Coincidentally, we discussed the issue of "Rock Art" on the List here a couple of months ago. I did a quick analysis and discovered some 60 WHS which contained Rock Art (graph or glyph). 24 of these were inscribed significantly because of their Rock Art (ie included in the OUV) and around 12 of these even had the "Rock Art" aspect in their name as, in effect, the main reason for their inscription. See http://www.worldheritagesite.org/RockArtWHS1.xls
So, we have a "Gothic Cathedral" situation here! How many to choose and how to differentiate. Age/period, "artistic" merit, geography/culture?
A number of the sites with significant Rock art are (probably?) going to get chosen for other aspects so ,even if we chose none from this category, the propensity of Mankind to express itself in this way would still be represented for our "Little Green Man". This is particularly the case for more "modern" examples where they form part of e.g African Cultural Landscapes. There are 8 African sites listed above and there are others too - e.g Lope.
My inclination is to choose just one of the most "significant" ones under this category and leave the rest to appear under CLs etc. The earliest seem to me to be the most significant - so it has to be Altamira.