It'd be easier to help if I knew what the error message said, but I have a couple suggestions:
a) The file may be either too large, or of the wrong type. To fix either of these problems, just upload the image you want to use it as your avatar on photobucket or tinypic, and once it's uploaded, right click on it, and save the picture. This will make the file smaller, and it will save it in jpeg format.
b) If you have a previous account on TeenPhrase, we could delete this one, and change your old username to match this one. That way, you would keep your higher postcount, and all your profile information, including your old avatar.