Perhaps it would work to simply save each layer individually, with alpha texture enabled on all layers but the bottom one, so anyone could "stack" them in *any* graphic-editing program.
They could be numbered, such as "bloodspider00.png, bloodspider01.png, bloodspider02.png, ..."
Such a restriction is not GPL compatible if I'm right..
"Non-commercial" can mean a number of things in my experience, which sometimes is GPL compatible and sometimes isn't.