Actually, the quality could be better. I've had a publisher actually use a web-sized old image and scan it at a very high resolution in order to use as a book cover.
(obviously in this case it was by agreement and we both were happy)
Mark, my images I chose for giving out digitally were images I decided I would never use myself for any other purpose. So I saw no harm in any way they could be distributed freely and was happy to take that risk. It might be worth choosing images carefully if you do go this route. I decided to use either older images no longer seen anywhere and which I'd never print (originals sold long ago andnot shown online anymore, plus I had an ok image for a small print but no higher resolution anyway); or in a couple cases I used images of paintings I later destroyed and never exhibited. So the latter ones were never in circulation in any form. For me this helped protect the images I felt had potential for future publishing or simply didn't want distributed freely.