I don't use this emulator so bare with me. Does it not work for the img files? If not then the easiest solution would be just to mount the image via external tools in the Start before setting of emulator configuration and then run the emulator. Then it can deal with multiple disc image formats. Otherwise work will have to be put in like creating a wrapper to launch different command-line settings based on file extension or to convert all your images to a standard format recognized by the emulator itself which seems to be bin, iso, img, cue, ccd, and mds.