Canvas scratchcard with random images

Hi, I am trying to combine a HTML5 scratch card similar to this:

http://www.websanova.com/plugins/scratchpad#.UNCbyIZBF8E

but want it to have about 25 random prizes "revealed", I can do a random image script in PHP and the Canvas scratch part, but can't figure out how to combine them. It would be great to not have to use PHP but whatever works. Anybody seen this in action or know how to do it?

thanks in advance