ill make an idiot guide in a minute, just gotta correct andrew 1st
isos= games backed up from disks
roms= games backed up from cartridges
neither are illigal UNLESS you download from internet, if you back them up yourself there are ok to use on emulators, roms are not used for eloader, roms are used for emulators that can be run on eloader, if you dont run the emulator you dont need/use roms/isos

Idiot guide
1 - check your disk tray (open UMD tray) and look up in top right corner for some writing
if there is none you have 2 options
a)degrade
b)eloader
if there is writing you have one option
b)eloader
im just gonna tell you how to use eloader for now, ask if you want to degrade

2 - go to noobz.eu and download 'super bock', which is the newest eloader, use exe if you are on windows
3 - plug psp into pc and run exe, follow the instructions
4 - download the game that you would like to play
5 - put the game unzipped/unrarred into the folder ms0:/psp/game where ms0:/ is you psp drive (for me its d:/)
5 - start gta, hopefully it should start eloader, if not, load the eloader
6 - browser to the game you want to play and press x to start

there you go, if you need more info i could try to give morre, just say what else you need to know