"Are they painting a picture?"
Translation:Maalaavatko he taulua?
It's actually ambiguous, because the possible subjects of a plural third person verb are "he", "ne", and any plural noun. For a singular third person verb, possible subjects are "hän", "se", and any singular noun. A singular 1st person verb, for instance, can only have "minä" as the subject.
"Taulu" is usually "painting".
"Kuva" is a generic word for picture without specifying if it's photograph, drawing, painting, engraving or what, but the verb "maalaavatko" here does says that it's a painting, "taulu".
"Maalaavatko he kuvaa" sounds like someone has a picture, a photo or something, and paints on it, that would be "Are they painting on a picture?"