My personal spray decal for the Dystopia mod for Half-Life 2. [link]
For those who don't know, Dystopia is a team-based, cyberpunk themed mod. In it, players can take options to make themselves invisible, in which they run around taking out the other team with their sword. This is called ninja killing.
And of course, Half-Life 2 allows you to paste your own images on the walls during games.