If you want to make up a cover for your NaNoWriMo novel (or any other story) but aren't feeling particularly inspired by anything, trying playing with this Fake French novel cover art generator.
Three of my results:
I really love the broken window and the open-book pages designs, and armed with my own camera I know I can come up with something very similar (or even better.) I'm not a huge fan of using museum art for book covers, but #3 might work for a more literary writer.
If you don't have a camera or aren't interested in photography, you can always look for similar images on stock photo sites like Dreamstime.com, BigStockPhoto.com or DepositPhotos.com (I found the cover image for my NaNo novel Lord of Midnight there.) Prices vary, but it's possible to find something (like mine) that works for around $1.00. Dreamstime.com offers free images, too, so if you're cash-strapped you might check there.