Minecraft wiki entry for Rube_Goldberg_machine, associated text: Make a dispenser shoot an arrow, snowball, egg, splash potion, or anything else that is shot as a projectile when put in a powered dispenser at a painting or item frame. This knocks it off. You can use this by making it fall onto a wooden pressure plate or into a hopper with a comparator next to it, or by having a comparator behind the block the frame is on, which removes the signal, if you have the comparator pointing into some redstone dust on top of a block with a redstone torch on the side, the torch will power back on. Or, you could simply use a target block, but this doesn't look as cool. Any way you do this will send a redstone signal to the next part of the contraption.