regarding the image i posted - judging by the rest of the imagery in that tradition, and the sorts of places in which the art was produced, it is very probable that the cultural group responsible for the images were shamanistic.
shamans were (are) spiritual leaders who make use of altered states of consciousness to establish and maintain contact with the spirit world.
altered states are biochemical phenomena, and the sorts of visual and somatic hallucinations they produce are often quite predictable.
in this image, we see floating human-like figures which is likely related to the sensation of an out-of-body experience, or of flying. the large eyes could indicate expanded visual powers. the flying objects around them may be what are called entropic phenomena - basically, the weird spots and shapes you see when you're on drugs. even the elongation of the figures is a well-documented somatic hallucination referred to as 'attenuation'.
the snakes and birds are common in this tradition - they probably had metaphorical significance.
it's hard to 'interpret' ancient art in this way, since it really consists of layer upon layer of metaphor, most of which we no longer have access to. there are clues to help us interpret, but in a sense you're right, it's all a giant guessing game
