just go to start /accessories/paint

then open your photo in paint, go to image/strech/skew
make your picture 25%, 50%, 75% the size it currently is , save it to the desk top,
go to the desktop , right click , hit properties . still too big ? do it again with a smaller % . too small? make it a bigger %.

now click on upload , since its on your desktop it will be easy to find .
Max. 100KB;
gif/jpg are supported