An avatar is the small picture under your userID. Here is how you can add one to your profile:
1) Make sure you are logged in
2) Click the "Profile" link in the top menu
3) In your Profile click "Personality"
4) Make sure you check "Use avatar"
5) Click "Upload avatar"
6) Choose an image from your computer.
NOTES: The image must not be any bigger than 100 x 100 pixels and 15240 bytes (15 KB).
If you can't resize your image try the following:
1) Visit: http://www.shrinkpictures.com/
2) Browse for the image and set the Custom max size to 100 and click "resize"
3) Save the resized image to your computer and then follow the steps above.
If you still have a problem please IM a staff member with a link to your image.
It sounds like the file size (in kb) is too big. One thing that always works for me is to convert the jpg to a gif or go the other way around. Could you send me the file as an attachment?