@acrylian: You're right that it's something I need to keep in mind. It hasn't been a problem so far but I added the link on each image page saying "Do you own this image? Please let us know here.", so hopefully if anyone has issues they can use that link to contact me and I can delete the image/s.
Thanks for re-adding it to the showcase, I'm honoured
@vincent3569: Yes the changing margins are done with jQuery, it's not a plugin but I wrote some code for it. Feel free to use it yourself, it's in http://www.dualmonitorbackgrounds/themes/mmg/js/jquery.mmg.js
I would prefer to do it with CSS since that is generally faster than JavaScript, but I don't think it is possible.