Well, in theory it's easy. You need to know how Retropie works, though (I suppose you're talking about a retropie image!). If you know how to access the ROM-folder, there's a subfolder for every system (snes, mame etc.), in there, the ROM's are placed. You could simply remove and add as you like. (best done via FTP, using Filezilla, WinSCP or whatever). BUT, you probably also enjoy having snaps, eventually bezel/artwork or even video snippets for each game. You can find all these assets via "games & packs" I think, but you'd need to check manually, or have a huge SD card and upload all ? That's basically my suggestion: Search for an image that you'd see as the one that fits your needs best, and then remove what you don't want. That's easier usually than adding stuff.. but as mentioned, if you don't care if some artwork is missing etc, you can simply copy files to the corresponding ROM-folder of the system you're editing. You can run a scrape afterwords of course, but that's not sooo reliable usually, plus if you want bezels etc. it gets a bit more complicated. But there's lots of tutorials around, you need to google a bit if you're stuck with a different topic…