Compare bash of screenshots

Here is where I am : I want to automate visual regressions. So, with CasperJS I took some sample screenshots at multiple sizes. I can use it too to take screenshots of the actual situation. All the screenshots have the same name, there just in two separate folders.

Question is : how can I automate testing? I'm looking for a solution a bit like compare from ImageMagick which gives me a superposition of the two versions and highlight the difference. But it should works on multiple files.

Any idea ?