I’m not sure which site came first, Thumbs and Ammo, or Zappers, but they both present the same idea. Take still images of from films of people pointing guns, and replace the guns with something else. In the latter of which used the Nintendo Entertainment System’s light gun (zapper). As you can see from the above (and below) photo(s), the former site uses thumbs up.
Thumbs & Ammo seems to have a larger, better selection of replaced images. It also looks like it’s been updated more recently too. But it also looks like you can make your own and submit them, probably why it’s been updated more.
Personally, I like the Zapper site better because Thumbs Up seems like a gimmick, and kinda takes away from some scenes, also, some of the scenes just don’t make any sense with the way the characters are holding the guns.