I found a few plugins.
They all work with mousedown to scratch the image, otherwise I think they are close what you are looking for. It shouldn't be very hard to change to mousedown behavior and make it loop automatically.
http://rabidflash.com/authoring/codecanyon/scratch/
http://www.senamion.com/blog/2010/03/30/jquery-jscratchcard-scratchcard-effect-on-image/
http://www.htmldrive.net/items/demo/880/Images-Scratch-Card-Plugin-jQuery-Scratchie