Yep, but remember the ratio of the image is just as important as the size. You don't want to squish it up or have it elongated horozontally or vertically. So crop it to a nice variable then resize it accordingly.
Example: The maximum resolution for an image on FAF is 150x150 pixels, but if the image is 500x600 you'll have to crop it to be square first then resize it to 150x150. Also the maximum file size is 50kb's, for most images simply saving it at a .png will keep the size low, however if an image is at 150x150 and you can't seem to get it lower than 50kb's you'll have to lower the quality a little. It's more complected but you can lower the quality without it being visually noticeable, not sure how to do this with GIMP myself, haven't used it in a while.
How about this, upload the image you want as your avatar onto
http://filesmelt.com/ and link it here, I'll do it for you.
<3
Edit: I realize FA is 100x100 and FAF is 150x150 so I'll make you one of both resolutions that you can use.