They're in JPEG format. To use them as layers - in GIMP at least - add Color to Alpha as white. You can then add them as detail to more complex textures. They might be usable as decals if converted to PNG - and have white set to alpha. I have the higher res originals still. When I can access a broadband network, I'll upload them as well.
They're Public Domain, so use them in any manner you see fit.
UPDATE: The high-res examples are uploaded.
