For a lot of artists, the amount of detail on characters is seemingly proportional to the amount of detail in the background. Assuming your artist loves his detail, but actually has to get the book out before the end of the month, some will actually use filters on actual photographs or prepared backgrounds. But these don't always mesh well, creating the paper equivalent of Conspicuous CG.
Occasionally that's replaced with abstract, quilt-like patterns, for artists who want to draw some kind of background so you don't feel as if you're reading a lightbulb but without messy points of perspective.
Naturally this comes off as a little less glaring if your story takes place at the actual real-life location used.
See also GIS Syndrome.