English speakers would never ask if you were painting a painting, as it sounds almost comically redundant. They would use either picture or a description of the painting eg portrait, landscape, etc. In this sense picture means taulu, not kuva.
Two reasons. First, use of pronouns in the first and second person, in formal Finnish, is optional. But more importantly, the correct pronoun here would be the basic form, nominative case, sinä, not the adessive sinulla.