I take it you have gone into your user CP (control panel) and gone to settings & option - edit avatar? you should be able to upload from your computer, but make sure it is not too big. The maximum size of your avatar is 80 by 80 pixels or 19.5 KB (whichever is smaller) any bigger & it won't upload.
Whatever graphics program you use on your computer go into image information & see how big it is, if too big, resize it. If you have trouble with it, let me know via PM and upload to your photo gallery ( they can be bigger there), I'll copy & resize it for you and send to your email for you to upload.