If you can do monochrome bitmaps, that would be the most preferred format. However, PNGs are great as well. I'm glad you're not doing hex, because I always have to do stuff with the textures to make them work with Elimination.
Monocrome? I thought it was in 3 shade grayscale.
BTW: which of the two themes do you like best: the upper (lighter) one or the lower (darker) one. I think the darker theme looks better on a computer, but I don't know how it looks on-calc.