I use texture overlays all the time for my digital illustrations. Some are watercolor washes, some are just aged paper textures, and mostly I've been getting them from stock sites like DeviantArt, or
WeGraphics My first attempt at my last painting, Messengers, died in the background stages because the the colors were darker than I wanted, but I decided to salvage it by turning it into a digital texture.
And it's available to download if you like using textures in your digital art, too! Feel free to use it for personal or commercial work.
The download is at my blog, packed up neatly in a zip file:
http://kirimoth.com/blog/index.php/2011/01/07/free-texture-download-bombay-india-ink/ Enjoy!