I have always believed that these were sacred images since most cultures that did rock paintings into historical times felt that way. While the religious significance of the images is lost to us, the fact that they were done with great difficulty is not. The most common religion found in hunting societies is a form of shamanism. Shamanism often works much like witchcraft, where intent gives objects their power. The images may of worked like a spell giving the shaman power over the images. Later with the invention of writing, magic often is worked with just a name or words written with sacred intent. Some may represent mythical figures and others may represent desired results. Many shamanistic beliefs involved journeys into the underworld to obtain what was needed for the tribe. A descent into the bowels of the earth to work your most sacred magic seems like it would have great power, both for the shaman and the tribe.
There is a cave in El Juyo, Spain, in which a rock with a natural fissure in it has been carved on one side in the figure of a cat, and on the other in the figure of a man. I believe that the people who did that, and who made the cave paintings were in such awe of the world around them, and of the mysteries of even their own being, that they dealt with it the only way they could - artistically. It is recognition, acknowledgment, whatever you want to call it, but why would they do it if they didn't have an inkling already that there was something beyond themselves, greater than themselves?