Without a doubt, the rock art of the San in Southern Africa has to be considered a genuine artistic achievement by pre-colonial Africans. /u/tlacomixle would be the person to go to regarding these things more intricately but from what I know from my own life in Africa, these paintings are both incredibly numerous and artistically varied. The pictographs have been found mostly in the rocky areas of land south of the Zambezi, but isolated cases have been discovered in parts of central Africa as well. About 15,000 sites have been identified as belonging to various groups of San. These cover a period of some 3,000 years, including right up to the present day. Some in the Drakensberg area even record San being hunted by men on horseback with guns, possibly the story of punitive "commandoes" carried out by Boer settlers on the violent frontier of the Cape Colony in South Africa.
All the various examples of San paintings are culturally and historically significant, and the recognition of the San rock art's importance has only increased in recent years. According to J.D. Lewis-Williams, the central image in the South African coat of arms derives from a San rock painting in the Eastern Cape. Other historians have called the San rock paintings and engraving the longest enduring tradition of art-making known to continue today.
Although the style of art appears relatively simple by modern standards, the detail in many of these paintings and engravings is simply incredible. Unlike the paintings made by other Bantu-speaking groups in Africa (who used their fingers to paint), the San used brushes made from small reeds and animal hair or feathers. This gave their paintings a delicacy which few other forms of art in pre-colonial Africa could achieve.
I was fortunate enough to see first hand examples in two parts of rural Zimbabwe which were dated back some 800 years and included twenty different species of animal, as well as humans in a variety of activities including hunting, herding animals, and eating. The animals are drawn to a physiological accuracy which would be impressive even for somebody to draw today. The two examples I saw were only a hundred miles apart and yet there were nuances and stylistic differences which visibly demonstrated how incredible these artists were and how much their art was individualised. Here are the two examples I saw. These photos are only small sections of paintings covering entire walls of caves and covered rocky outcrops. In the case of the latter, it is located half-way up a granite kopje, some hundred feet above the ground, accessible only by scrambling up a steep slope and squeezing through a narrow gap. That the San people took the time to bring charcoal, ochre, animal blood, plant extracts and other pigments to these places in order to record these images is significant. Unlike other forms of rock art in Africa, these were often not done for any practical purpose either. They were purely expressions of artistic abilities or to record cultural practices.
I hope this helps answer your question as one example of the artistic achievements of pre-colonial Africans. There are many books, blogs, and websites out there which talk more in depth about the history and provide more detailed analysis of the rock art itself. Any questions, please ask and I shall do my best to answer.
I'm an artist with a special interest in African art. If this contribution isn't scholarly enough, please remove it. My books are at home, and I can provide a bibliography when I return.
Pre-colonial Africa had some amazing metalworkers. Metal artwork lasts longer than wooden ones and is easier to date than stone artworks.
One favorite group of works of mine is the Benin Bronzes. They were a highlight of my visit to the British Museum.
A similar body of work that I find extremely moving and disprove the stereotype of African artists as incapable of naturalism or sophistication are the Ife heads from Nigeria They affect me in a similar way as Greek or Roman classical portraits.