How do I convert an ID to an compatible one? (non-studio)

Hi, I'm making a OS based on Roblox. But, I'm starting to make custom stuff. For example, custom wallpapers by typing/pasting IDs to set the wallpaper. But you'll have to find the actual image, if you try to subtract the last digit by 1, it will work but not on all decals. It's because you'll need to find the actual thing. I can't seem to find anything in the F3X source code. The best I can think but currently can't do is to subtract the ID by 1 until it finds the owner username on the image like it was on the decal. It could be unpleasant as that person may upload 2 different types of stuff. So does anybody know how to convert an ID while not in studio?

Thanks in advance.