So, you want to post an image in a Forum, and have it change randomly each time you refresh?
The easiest way I can see to do that would be use PHP to create an image, and randomly select which one to use. PHP basically has all these functions built-in, so it wouldn't be that difficult.
Look up some tutorials on how to create images with PHP, and then how to randomly select a value from an Array, and then go from there.