Once you've mastered the basics of Photoshop, you may want to start playing with paths and vector-based art. There are several sites I frequent that off free vector-based art. Because vectors are created using mathematical formulas rather than pixels, the quality of the images is much greater.
Bittbox offers a wide range of vector art, everything from watercolors to flowers to gears. The site often offers accompanying packages of brushes to match the art.
Vecteezy is a fun site, billing itself as "stupid name, cool vectors." If you're looking for variety, this should be your first stop. Because the work on this site is submitted by designers from all over, you'll find tons of different subjects and styles.
I'm getting ready to dowload T. Scott Stromberg's Leafy Pod, myself.