Photoshop has a lively community of people who make handy textures, brushes, tutorials and patterns available for free – sometimes as an excellent way to introduce their commercial products, sometimes just for the love.
One site I go to when I need to pose figures artistically against a gritty wall or rolling wave, and I just don’t have a picture of the right wall or wave on file, is Texturemate.
Texturemate has hundreds of high quality, well made, nicely organised and all-round very useful textures. And, all free to use even commercially.
A Little Art 