http://www.dickblick.com/products/speed ... m-cutters/Get one of these. I have one and used it for making prints out of Linoleum.
If you can have your image drawn onto the foam and use your speedball to cut out the lines, your impression will be the negative space. I then would leave it like that or take a black sharpy and color in the space you cut out. Be careful with the speedball though the blades are extremely sharp and I almost severed my thumb off when I dug into the linoleum. It slipped and went right into my thumb. Hurt like hell and I passed out at the amount of blood coming out of my hand. That would work in your case thought for tombstones.