I'm a bit confused about what you mean by afterimage; are you saying something remains on the screen after you close the game? This asset isn't doing anything nonstandard (it all comes down to drawing a fullscreen surface in a Draw GUI event), so if you find your game isn't closing correctly, this might be more of a bug on YoyoGames' side or perhaps a driver issue. If you can provide a bit more information to sittingduckgamesco@gmail.com, I can at least ascertain whether or not I can resolve your issue with a patch to the asset pack.