I find that doing a google image search works for many things.
Search for sprites plus the thing you are lookign for (eg - alien sprites). For backgrounds, a search for "theatrical backdrops" yeilds some interesting results.
http://reinerstileset.4players.de/englisch.htm
Lots of sprites here - ignore the "3d meshes" bit and concentrate on the "graphics" bit.
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Also this site might help....
http://charas-project.net/resources.php?wa=0&lang=en&area=Backgrounds&howmany=10&fsearch=
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For video game sprites you can use the shyguy kingdom
http://tsgk.captainn.net/
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Or you could just search for 'Animated GIF' inn Google if you want something that looks like it's moving for your game or animation.
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