Assuming your FurAffinity pages display for you the same way it does for me (on a desktop monitor) : in the upper right of the page...
"Settings" / "Manage My Content" / "Avatar"
and then it gives some instructions from there. If you have an image in mind and on hand already, then you may still have to edit it yourself to get it to fit to the size that FA wants (100x100 pixels or smaller), but that editing should be pretty simple to do on even the most basic of art programs
(speaking from experience in that regard).

I have one but it's not working

(I haven't changed my FA icon in years, but...)
https://www.furaffinity.net/journal/7413188/

Is your picture already within the accepted size range, and is it one of the file types explicitly listed on the "Avatar" page? Also... is it a screen capture? For whatever reason, I've never been successful at uploading one of those here, either.

simple, use MS paint, the quality will already be shit to begin with and you can then crop it to make it within 100 by 100 pixels