As a Roblox developer, it is currently too hard to retrieve an image url from a decal id from a script. There is no clean method of getting it currently. Right now you have to write code to subtract one number from the decal id and verify it and repeat until you get the correct image.
There is a way to get the image url with InsertService, however this is not viable for other people’s decals, since InsertService restricts calls to only your assets.