The whole picture is here, but the stain is not too visible after I had to readjust luminosity curves (original photo was very dark...)

The stain in itself isn't very interesting : no fancy splashes, just a round brown patch. I played a bit with it by surrounding it with that circled pattern in the right bottom corner before ending the work.

The main problem was from the paper curling up due to humidity... That's not practical at all when punching myriads of dots on a that kind of small surface :D