It has been a long time since I loaded my avatar. I think, and anyone who knows better or knows if I'm wrong about this jump right in, I think you have to make the changes on your computer before you upload it. Are you saving it as a .jpg file? I'm pretty sure that is what I used. I thought if the picture was 80 x 80 pixels it would be small enough. You could try making it a little smaller. The only thread I could find that has any discussion about changing the picture is this one
but the instructions regarding photobucket are too old, but it does have an editing program. I'm not sure if changing a picture to a .tif file or a .gif file decreases its size or not.