Official Firmware 3.72 has a built ISO driver but it can't be used to play ISO and CSO rips. It's one of Sony's ways of fighting piracy. To play ISO's, your PSP would have to be running a custom firmware. CFW's have a built in ISO loader. I recommend 3.71 M33. It has better homebrew compatibility than 3.72 HX, has much more features and practically no bugs. It even has better PSX compatibility thanks to the new popsloader.
