This weekend the Liberated Pixel Cup started its art contest, which will run until the 30th of June. You can check out the style guide and the existing assets here (includes a interactive HTML5 demo).
Reasonable prizes for the winners and runner-ups might motivate you to contribute, but judging will be only after the second phase of game prototyping, which ends on July 31st.
But even if you don't win, you can be sure that your contributions will be a very valuable part of the resulting FOSS game content pack, so every small addition counts.
Good luck!
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