The boys made their own window clings earlier this week. I taped a clipart image underneath a piece of parchment paper. Using puffy paint, they painted on the parchment paper. (You have to paint pretty thick.) We let the paint dry overnight.
When they were dry, we removed the paint from the parchment paper. I don't think we painted thick enough because there was still a thin layer of parchment paper on the back of the cling. However, the front side of the cling stuck to the window really well. (They haven't fallen down all week.)