MSPaint is fine. Others would use IDraw. Either way..
Each file is 144x384 pixels, and each "frame" is 48x48. (Obviously, this means 8 rows, 3 collumns.) From there, you would just draw whatever you want, with 3 frames of animation for each pose. Open up a current RTP one and use Save As to make a copy somewhere so you can mess around with it.
Make sure it's 256 colors! (If you're having trouble importing it, just Save As a 256 color bitmap in MSPaint.)
Don't forget to set up your battle charset when you're done (Animations 2 tab in the Database). If you don't know how to set those up, look here: http://zelaron.com/forum/showthread.php?t=27929
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