Photoshop would probably be your best bet if you want to layer them so you can see the details from images underneath. Using PS you could vary your opacity of layers. Mind you 30 layers will look pretty messy. An animated gif might be a better way to go.
The Gimp is the best free program. It's similar to photoshop. It will also do animations. Although, using Photoshop and The Gimp are not the best ways to do animations.