If you look at the info when adding an avatar it says and I quote
The maximum size of your custom image is 80 by 80 pixels or 19.5 KB (whichever is smaller).
I found if you are using photoshop etc it is easy to get down to 80 x 80 but the 19.5kB is a bit tough
What I do is load it into paint and save and this reduces the file size to an acceptable level.
Hope this helps.
DoY