I use watermark, its easy just create .htcacces on the folder u save the pics and copy/paste:
RewriteEngine on
RewriteRule ^([^tn].*\.(gif|jpg|png))$ /imported_images/watermark.php?image=$1&watermark=watermark.png [NC]
But with this option I had a lot of complaints of my forum users, because they dont like watermark their images
I would like to know how to watermark with exceptions such image size, or file size,
Best Regards