Where do Artist pictures i add end up, being stored on a Mac ?
In the Roon database that the Roon Core is using.
To my knowledge, they are not directly accessible from the file system as they are stored using file/folder obfuscation.
Not a Mac user, but I think it depends on how you add the picture. If you past a URL, the only thing stored is the link to that image stored in the Roon core database. If you upload an image by browsing, clearly that picture must be on local media storage, presumably the hard drive on your Mac.