How to create a graffiti in Left 4 Dead 2

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I was playing Left 4 Dead 2 and I saw a player paint a graffiti on a wall (I also saw it done on Team Fortress 2)

How is this done? What is it purpose?

Is it different on those two games or is it something at the engine level?

Best Answer

The place you configure your sprays is in the options menu, under multiplayer.

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From this place you can import an image of the proper dimensions, which will show up when you use the spray command (default T) in game.