Rubella, I don't know what program you use to make your avatars but I personally prefer Paint.nethttp://www.getpaint.net/
I start by opening a picture (obvs). Then I use rectangle select and change it from normal selection mode to fixed ratio, and set the ratio to 5 x 7. Then I draw the rectangle over what I want as my avatar. I use the Move Selection arrow to center it up all pretty. Then I crop to selection, then resize to 100 x 140. I nearly always save as PNG and you can move the quality slider a little down if he filesize is still somehow too big. Tada!