For animated avatars, you need to use someting like Gimp (Google it) to resize it to 75x75 or smaller
For non-animated avatars without transparency, you can upload it at any size and the forums will resize it automatically to 75x75 and to .jpg format
For non-animated avatars with transparency, you'll need to resize it to 75x75 in Paint.Net, Photoshop or possibly MS Paint, if you upload it without resizing it, the transparent parts become black and the quality will be ruined a bit (Due to the forums turning images bigger than 75x75 into .jpg's)