Rock-painting - simple and interesting kind of art, I started create rocks in 2013. Rock-painting is popular enough now, and you can find a lot of resources on this topic (sites, books). The most essential thing - imagination, because when you look at the stone, you can imagine, what it looks like and what kind of animal can get out of it.
I use rounded and smooth pebbles, draw with tempera and acrylic paints and cover with a glossy varnish. All rocks are up to 10 cm on the long side, and on the average 5-7 cm, so they are easy to put on the palm. I put my soul into every creation, treat them with care, because all these little creatures look like alive for me.