I discovered some time ago the .pbp file format. In fact it’s a compressed format meaning that you could somehow compress your isos to let them use less storage space (excellent if you only have a sd card). Also in case of multi-CD game then all of them will be packed and compressed within a single pbp file! There are tons of how-to’s on the Web. The most famous tools are PSX2PSP v1.4.2 and PsGUI300beta.
Hope this helps!