Below is a list of avatars that a user with a Nitrome account is given and can earn. Avatars allow a user to customize his/her profile.
With the announcing of Accounts, Nitrome planned on having an avatar based system. This system allows for the user of an account to use an image of a Nitrome character as their avatar, but does not allow them to use an image they created him/herself, likely because how this type of user generated content is mostly offensive, as was the case with Hot Air Balloon Maker custom balloons.
To get around this, pre-made avatars were created. The user of an account is given a set amount of avatars upon creating their account, and can get more by performing finding hidden presents around Nitrome's site, and in their games.
Pink presents with gold ribbons were hidden around Nitrome's site, and in their games. Players could obtain them by clicking on them or touching them with their game character. When a present was taken, a message saying "New avatar unlocked!" would appear at the top of the site's screen. The present would then appear in its spot on the user's avatar page. The user could then go to their avatar page and click on the present to open it, revealing the newly-unlocked avatar.
During the month of December 2013, Nitrome had a special avatar event. This event lasted from the first of December to the twenty-fifth, Christmas. Every day, a new avatar challenge was shown on the special Christmas avatars page. If the player then unlocked the avatar and went to the avatar page, a chocolate engraved with a present would be shown over the avatar unlocked. Once clicked, an eating animation would be shown, uncovering the unlocked avatar by eating away at the chocolate.
A regular present
A big present
A big present variant
A small present
A small present variant
A 3D variant
A present on the avatars page, upon finding an avatar
Hint: This is the present you have been searching for!
Office Trap Worker
Escaping Worker from our game Office Trap
Type 'present' in the search box on the Nitrome website and press enter. A message will congratulate you for finding the hidden avatar using the secret keyword 'present'. Click on the present that appears.
Hint: A perfect score on level 4 will reveal this icy present, but don't crush it!
Hero from the game Lockehorn
On level 4 of Lockehorn, smash all the pots and collect all coins before crushing all the snow spirits. The present will come down as the altar opens. If the block crushes the present, it will no longer be obtainable.
These avatars have currently not been released and are completely unobtainable as usable avatars. The only proof the avatars exist is that they have been uploaded to Nitrome.com, and as such, are only viewable only when the avatar's image URL is gone to.
An avatar of Blue enzyme from the Test Subject series 
This avatar was uploaded on July 12th 2013 with the first batch of avatars, and was also present in an image advertising accounts. It was never made accessible as a usable avatar, however, as two other avatars uploaded at the same time and not used yet, it was assumed these avatars would eventually be used.
On December 1st 2013, Nitrome stated that this Cuboy avatar would never be used as it was decided that Cuboy would represent Nitrome.
Nitrome first hinted at the Christmas Avatar event on November 22nd, referring to it as a "We have a few other top secret things ... the first one should be revealed on the 1st of December!" It was later mentioned again on November 29th when Nitrome made a blog post about the upcoming event, and released a blurry picture of it. On December 30th, Nitrome released the Christmas Advent Calendar, along with a blog post about it.
The Christmas Avatar Calendar is a virtual Advent Calendar that allows players to hunt for a new avatar that is released each day. The calendar (like all advent calendars) went from the first of December to the 25th (Christmas), and on each of those days a new avatar and clue would be released. Each square on the avatar calendar had the number of the day on it. Once the day came, the square would change to an image of a lock, and hovering the mouse over the image would show the hint to finding the avatar. Once the avatar was found and obtained, the locked square changes to a chocolate with a present on it. Clicking on this chocolate will cause an animation to play of it being eaten away, revealing the avatar.
After the first week of the calendar took place, Nitrome created a blog post about how pleased they were the outcome of the event so far.
Coming in the future version for iOS and Version 1.0.0 for Android, Nitrome will be adding a new feature that allows for people who buy the game to be granted eight new avatars in their Nitrome account, these avatars being Icebreaker themed.
When the updates comes, a link will be added in the game which allows the player to log into their Nitrome account, which will unlock the eight avatars. These avatars will only be able to be unlocked through this method, people without the game will not be able to unlock the avatars. 
July 12th 2013
Avatars 1-40 were released.
September 11th 2013
On September 6th 2013, Nitrome announced in their Nitrome update that they would soon be releasing new avatars to find and collect the coming week (September 9th-September 13th being the week they would be released). Avatars 41-48 were released on September 11th 2013.
When changing an avatar, the user of the account clicks a URL to change their avatar to another one, this URL being http://www.nitrome.com/profile/avatar/index.php?a_id= , with after the "=" sign being a two digit number.
Placing after this equal sign in the address any letters, symbols, the numbers one, three, eight, thirteen, or any number not already assigned to an avatar, then going to this URL twice, will cause the player's profile avatar to become completely white, their avatar when looking at their profile page to be the avatar that they get when they have just created their account, and all the comments they have made on the comment sections of game pages, these comment will disappear.
Going back to a regular avatar will cause their comments to reappear.
Invisible present glitch
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This glitch has only been found in the game Rustyard. The gift on level fifteen may go invisible after the present from level six has been collected. Rusty will still walk on and interact with the gift as if it were a normal block, though. This glitch was fixed on September 24th 2013.
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Beginning on level 25 of Fault Line, leaving or completing the level, then going back to level 25, will cause the present to appear. However, exiting/completing the level without collecting the present, then going back to level 25, will cause the game to not fully load the level, and instead load a white background with Zapo spawned out of thin air at the top-left corner of the screen, and close to the middle of the screen having two nodes; the player's level cursor is also present, and works regularly.
These two nodes are inside an invisible box, which cause the player's cursor to disappear when inside the box, and to reappear where the player's cursor is outside the box. The nodes cannot be clicked. Menu music for Fault Line plays in this situation. Despite being spawned, Zapo cannot be moved.
The pause, music, and sound effects buttons are present, and the pause screen works fine. Exiting through the pause screen back to the menu causes a never ending purple screen to become present, forcing the player to reset the game page.
Sometimes, when a avatar is obtained, it will give the player a message that they are not logged in (even though they are), and the present will not disappear. However, if the level or game is reset/refreshed, the gift will not be seen, and the player will have the avatar.
Unreleased obtain methods
During the time of the Christmas advent calendar, some methods of obtaining an avatar were publicly made available before the respective avatar was made available.
Hot Air Jr
The cat and owl avatar was unlockable before it was released, to pre-unlock it, the player had to push Hot Air Jr into the gap in the ceiling at the end of level 11. Once this was done, a notification would pop up saying that the player has unlocked an avatar, despite the fact there is nothing there. When the avatar was released, it would already be in the player's inventory.
Bad Ice Cream 2
There is a glitch where the player could find, but not obtain, an avatar gift from level 13, even though the avatar had not yet been released (at the time).
The avatar of day 15 was unlockable before it was released. It could be found from any multiplayer stage by moving ice blocks and making them to collide into walls. The present should drop down from the sky. If players collected the present, they do not need to do a task for day 15.
By clicking on the recycle bin on the front screen of Oodlegobs before clicking the "click to play" button, the trash bin would pop up a window that has an outline of a present. This was available even before the proper avatar was released. However, after the avatar was released on September 11 2013, the present became filled in. Although, once the present is clicked, the trash bin will go back to how it was before the update.