Many skins provide a relatively easy way to include background images, particularly for the thumbnail pages. Regular images are usually too distracting, drawing the visitor's attention away from the album images. What works best is to use a repeated texture image, often the subtler the better.
Some years ago, user rfm2 put together a collection of background texture files, but the links to those files were stale, and the thread was deleted. However, I have most of his stuff on my PC, and I've added some things to the collection over the years.
I haven't checked all of these (not by a long shot), but I know that some of them don't "tile" smoothly. In other words, when repeated over the entire viewport, they don't create a single texture, but retain visible borders between one repetition and the next. You'll have to experiment!