The only reason Holmes "remembered" anything about the van Buren Supernova was because it was mentioned in the planetarium where the Gollum killed the professor. As she is fastforwarding the video. He also asked John if he heard it too. That is the only reason he was able to deduce that it was the van Buren Supernova. He remembers things he hears when it's relevant, but discards the rest. It can be safely assumed that once that information was passed along, it was "deleted from his hard drive." As far as why someone would paint it in there was probably because a.)the painter wanted to be recognized (This is possible though unlikely.) and b.) If the lost Vermeer was truly lost, then he can only paint by description and was probably unintentional. or c.)Moriarty is the one who had the artist paint it in there so that Holmes could figure it out. That's the whole point of the episode anyway. Moriarty wanted to see Holmes shine, to see what he could do, so that he could set the plans into motion to beat him in Reichenbach. He basically wanted to know Sherlock's limits.