I presume you'd have to be able to get the files out of the PSP ISO first, right? I don't know anything about it but I'd guess that PSP games are encrypted, so its not exactly easy. The way I see it, you have to do the following steps. If you can't do things higher on the list you can't go any further, so figure out step 1 and 2 before moving all the way to step 5 or 6. Hopefully you can find the tools online to do each part in sequence. I also assume you're talking about 2D graphics. It's inconcievable to try to extract 3D graphics.
1. Decrypt the ISO.
2. Access the file system to get files. There's a good chance the ISO isn't encrypted but the individual files are, which makes it harder.
3. Identify which files are images. The images are probably larger files but not very large (the largest are probably audio or videos)
4. Determine the format of image files.
5. Extract each image from image files, if the image files contain more than one image (like animated GIF files do)
6. Decompress each image. Fortunately modern games rarely use lossy graphics and there's less need to disguise the files on a game system than on PCs, so there's a fair chance the images are some simple format like BMP or raw pixels, or in some super-standard format like PNG.
