There was Tekhne, a minor deity of art, craft, technical skill. Although you won't find much more info than that on her. Not specifically painting, but I think it would fall under that. Hephaistos is another, not specifically painting, but of crafts, sculpture, that sort of thing, might be considered more visual than, say, Apollon, who tends more to the music and poetry side of thing (but, who would also still be appropriate to petition in this case).
I sing of you, blessed, night-winged Dream, Messenger of things to come, greatest prophet to mortals, in the quiet of sweet sleep you come silently and speak to the soul.