bob n
Take a piece of scrap sheet rock, whatever size you need or can get. Break the edges with a hammer or pliers to make them irregular. Peel off the paper cover from one side of the sheet rock and draw a primitive cave type picture or animal on it. Scratch the outline and/or simple relief of the animal. Color or stain the sheetrock to look like real rock. Put a hanger on the back.

Clare Lewis
You could get a piece of slate rock or to simulate rock -- use plaster or paper mache on a wire frame, and then paint a scene in acrylic paint. Generally, the colors used were reddish brown, black and yellow ochre. If you wanted to get the colorants and make a wash, they would be red iron oxide, black iron oxide and yellow ochre. In the link below is a virtual tour of the caves at Lascaux in France. The paintings are mainly of animals: horses, cows, bison, and deer, but also include hunters. Hopefully this will give you some ideas. They are pretty amazing! Good luck.

dragon
some Aboriginal art reminds me of cave painting. if you want to do 3D, think of mask, boomerang or poles (dupun: decorated hollow logs; they have a strong cultural significance). see the site paintedcountryaboriginalgallery.com.au wow, now I see you're from Australia... (to me cave painting is Lascaux. I'm from Europe...) so my answer won't be giving you much new info I guess?