Smart rule for deleting images that are not used in markdown documents?

Thanks so much to all of you! Currently my imported markdown images are saved in the assets subgroup and linked to as assets/xyz.png. For telling the truth I am very unsure if I would better use item links in my markdown documents (currently I am not using any third party markdown editor and therefore item links would also work. Would it even be possible to convert the non-item links to item links? do I need scripting for that (no scripting knowledge on my side)?