As far as I know, it went roughly 'pictures were of animals and hunters, males were hunters, ergo artists must have been male'.
This is off of wikipedia: 'Henri Breuil interpreted the paintings as hunting magic to increase the abundance of prey.'
'R. Dale Guthrie ...hypothesizes that the main themes in the paintings and other artifacts (powerful beasts, risky hunting scenes and the representation of women in the Venus figurines) are the work of adolescent males, who constituted a large part of the human population at the time.[23][verification needed] However, in analyzing hand prints and stencils in French and Spanish caves, Dean Snow of Pennsylvania State University has proposed that a proportion of them, including those around the spotted horses in Pech Merle, were of female hands.[24] '
en.wikipedia.org/wiki/Cave_painting
I have got what looks like a brilliant book on cave art, that is on my teetering to-read pile (The Mind in the Cave). Maybe I'll crack it out!