+5  A: 

You can try ImageMagick:

convert input.gif -transparent white output.gif

For more advanced features you should try something like image masking or do it manually using any image library, setting the pixel values to something with a high alpha value.

See also this thread: http://www.imagemagick.org/discourse-server/viewtopic.php?f=1&t=10665

 2009-06-14 15:52:01
using gif will create jagged edges, however using imagmajick is a good answer nonetheless, just use PNG output though and allow a bit of anti-aliasing at the edge colors.
SpliFF  2009-06-14 15:54:23
Also, ImageMagick has bindings for nearly every language, so you can use the ImageMagick library from within C
rascher  2009-06-14 16:23:01

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