I had this same problem and due to the fact that I don’t use the programs much, I didn’t want to pay for a program. So I decided to try something and it worked for me. Go into your es_systems.cfg file and go down to the PSX section. Under the supported formats (I think that’s what its called) add .iso and .ISO. After that my iso’s worked. Havent tried all my games but the ones I did try worked.