Renn starts out as a painter apprentice. It's during these years that he starts to realize his abilities. In fact one sign is that he is trying to get the right shade of green for eyes in a portrait, that is called Imager's Green because only Imagers can make it, and suddenly the portrait has the right color. Later on he paints portraits of the Masters and even draws quick sketches of suspects when he is a patroller. The portraits take weeks or months to finish. So why doesn't he just Image the portraits?
He is shown to be a very strong and talented Imager, in later books it's shown how strong, but he never once seems to grasp that he could just Image a painting. Following the rules of the book it wouldn't even be taxing if he laid out all of the supplies the same as if he was painting it himself.