After tearing hair out this is what I found….
1) Set pixel size to 80 x 80 max. (If rectangular then the biggest side must be 80) So to get the largest avatar you need a square picture.
2) Max file size 6KB
3) Actual image dimensions makes no difference (i.e. cm/inches)
3) There are three allowable formats jpg, png, gif
4) I found with Photoshop it was practically impossible to create a jpeg smaller than 6K. The easiest file format to get small enough was gif. (Switch off transparency)
After that it is easy!
Ray