Something that's painterly resembles or shares the qualities of a painting. You could admire the painterly style of your four year-old cousin's crayon picture of her cat.
Painterly technique might include brush strokes, use of color, or the particular texture of a work of art. Something with painterly qualities might not even be a painting — it could be a photograph or a drawing that mimics the style of a painting. This adjective can also describe a person who see things the way a painter might, like your friend who snaps photos on her phone and is praised for her painterly eye.