Because TeamNF didn't bother to do this themselves, I will do it for them.

Avatar Guidelines.

Avatars should be 76 pixels wide by 72 pixels tall. They need to be in .gif or .jpg format, animated gifs seem to be disabled. Maximum size allowed is 100 kB. If your picture is too large, it won't be allowed. If your image comes out with the colors all messed it, it is probably being auto-compressed. To improve the quality of your image, make sure that the image you upload is already the proper dimensions and size, so that it doesn't get messed with.

This guide will be amended with more accurate information as it is obtained.

I would also like to point out that 100kB is extremely generous considering the dimensions of the avatars. My avatar for example, is only 4 kB.