GIMP

GNU Image Manipulation Program

How to remove pure white

Quote from Dalestan (http://www.tt-forums.net/viewtopic.php?p=805832#p805832):

Well, some. Open each image in GIMP. Use the Color Select tool (Shift-O), with the threshold set to zero, to select everything that's #FFFFFF. Use the Fuzzy Select tool (U), with the threshold set to zero again, to unselect the contiguous background block, and then use the Paint Bucket (Shift-B) tool to fill the entire selection with #FCFCFC.

This, in a couple minutes, fixes all internal whites, without concealing any warnings about sprites that include the background.

Then save, encode, and manually fix the remaining ones.

Links

Main Website - http://www.gimp.org/
User Manual - http://docs.gimp.org/