Hi Mr Thicko (oh, sorry, you didn't actually *mean* that, did you . . .)
Slap me down in flames if this is wrong, but i think you:
log on go to My account (top line of any page) click on Account settings (left hand menu) scroll down to Your avatar then Browse to the image you want then Upload. It gets indigestion if your image is too big (in file size) but I can't remember what the limit is.
Hope that works, we certainly can't have a Fed with an ill fitting avatar.