There's only one solution to this, and that's customer education. I have to turn down 2-3 jobs a week because the customer doesn't have good art and won't pay to have good art created.
I had one customer send over a blurry photo that was 200 x 250px at 72dpi, and get really upset because I couldn't blow it up to a 12 x 14in picture with crystal clear resolution. I finally put a section on art guidelines on my website, and make all prospective customers read it - it's had a huge impact on the quality of art I get sent to me.