Stig's Art Grabr is a userscript for grabbing high resolution album cover-art from various sites. Can also be used as a bookmarklet/favelet.
For more information, see https://greasyfork.org/scripts/20771-stig-s-art-grabr
I don't use this tool every day myself. Or every week. Not even every month. And some of the supported sites I even more seldom visit myself. So DO give me a tip if it stops working on some of the sites which is supposed to be supported. I might be slow discovering it myself if nobody tells me...