The stamps must be in SVG or PNG format, if in PNG format, they should fit in a 500x500 pixels box.

The files must be under a free license, if you are the author, you must declare the license you want to release your files, if you are not the author, then you must prove that the files are under a free license i.e. A link to where the original author specifies the license.

You must also provide an English text accompanying each image that will display with tuxpaint, optionally you can provide too a non English translation.

Optionally too, you can provide accompanying sounds (sfx and/or descriptive sounds) in ogg format

See http://www.tuxpaint.org/help/ and http://www.tuxpaint.org/docs/html/ADVANCED-STAMPS-HOWTO.html