Everybody's seen JPEG artifacts, even if they don't know what they're called. They're what you get when a graphics file (saved as a JPEG, naturally) is blown up to a size bigger than that for which it was intended.
What you get are nasty, blocky blotches and lines in the picture, causing general ugliness and woe. It's the kind of thing you'd expect to see in the sort of cheaply made magazine that gets all of its pictures by grabbing them from Google Images, no matter the quality or copyright issues.
It's not, however, the sort of thing you'd expect to see on professionally published book covers. Unfortunately, it seems to be becoming more common. Look at the author images on the inside back flaps of the otherwise beautifully designed books put out by Hesperus. And even, worse, take a look at a recent novel published by Grove Atlantic UK, Serpent in Paradise.
It looks fine at this scale, reproduced on a 72dpi computer monitor. However, the lovely cover painting by Henri Rousseau (viewable here) falls apart when you look at the actual book. The publishers have used a low-resolution copy of the image, and stretched it to too large a size. The result is, unfortunately, rather ugly. Here's an example of how it looks:
This sort of thing does the author, publisher and designer involved no favours at all.