This is a good listening exercise that can be used as a warm up for descriptions. The innate competitiveness of most students impels them to listen very closely and do a bang up job of creatively rendering the verbal description. Narrow the finalists down through small group committees and then judge the finalists as a whole class. Rather than an art show place the students in pairs and get them to repeat the procedure, taking turns rendering a funny face. For the thrill of the moment at least they'll complete it in English.
Bad habits die hard however and are best dealt with from the start.
Be tough with the students. Don't speak to them in their native language. When students don't correctly use them just point to the phrases and wait them out. Most students are very uncomfortable with a sudden hush. There should be zero L1 in the classroom.