Go to your profile page, and at the bottom is the Avatar Control Panel...
You can upload a photo to your choice of storage website (such as imageshack or photobucket)... and paste the link in the top box...
alternatively, you can use the bottom box and browse to an image on your hard drive...
the image width can be no greater than 125 pixels, the height no greater than 200 pixels, and the file size no more than 95 KB...
I am guessing that the size is most likely your problem...
Cheers,
Daewoo