Idea generators may be able to help you out when you have trouble coming up with something to draw, results may vary.
Not the most useful generator, but you may just find a saying that makes a light bulb go off in your head.
The so called 'visual brainstormer'. See the site for more information.
Maintains a vast number of generators of high quality covering many popular genres. The community has art contests open to most mediums that are made with results from the generators.
This website specializes in generators for RPGs (like Dungeons and Dragons) but they are easily suited for ideas for art as well.