mobile image for Image Show GK4

How do I set a specific image in this module for a specific display size (i.e. mobile device).

Sadly, there is no direct way to do that, but You could do a little trick.
Create multiple ISGK4 slideshows modules, place them in same spot, use different image size for each of them, and then use suffixes .onlymobile, .onlytablet or .nomobile, .notablet to display just the correct module.