I think I've previously explained how we, who do not have the artistic talent to free-hand draw everything, get things onto posterboard: we trace line drawings (from coloring books or the internet) onto overhead transparencies, using wet erase markers. Then we use an overhead to project the drawing onto a bulletin board, where the posterboard is stuck up with thumbtacks. We use pencil to trace the projected lines onto the posterboard, then we take them down and lay them on tables and paint them with poster paints. People with all levels of artistic skill starting from
try to color mostly inside the linescan participate. Once the paint dries we take black magic markers and draw in the outlines, which helps to cover up the sloppy edges and also makes everything look better.
For the teen room walls, we did up this series of five comic strips, one of which goes with each day's lesson.