paint (something) onto (something)
To use paint to create a particular, picture, pattern, or design on something or some surface. Can you paint a big dinosaur onto Timmy's wall? He loves dinosaurs. I'm going to paint a cool fire design onto the side of my car.
paint something onto something
and paint something onto apply a design or picture to something, using paint. Joel painted the portrait onto a large sheet of plywood. He painted on some leaves and flowers too.