Have you tried such things as Pinterest or similar ? I opened a browser window on Pinterest, typed "angus mcbride art" as search criteria, and got a full page of them. Apparently, he also did historical art.
One of the links went here : https://www.les-ailes-immortelles.net/forum/viewtopic.php?f=120&t=10205&p=399268. It might be useful for Middle-Earth artwork (there's a map with the location of the apparently 29 artwork he did for MERP, followed by images of the artwork.
Now if you need high-quality image files, I don't know.