In Photoshop, make sure the file is 8-bit (not 16) and the color space is RGB (not CMYK or anything else) - if that's not it, also check that you've rasterized any layer effects (glow, stroke, drop shadow, etc.).
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Mark Spencer
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press
http://www.applemotion.net