Of course you have to make your own texture in an 2D app if you want to add an custom graffiti.
You can make graffits with different methods probably the easiest way is to make your texture in an 2D app (one that supports alpha layers, tga, bmp or dds fileformat) The texture has to be an power of two (128*128,256+256, etc). Import the texture to mylevel (I assume you know how to import stuff to your maps) make a brush sheet with the same dimenions as you texture then go to the face properties (right click somwhere) and set it to one sided and then convert to a static mesh. You can now use this mesh like any other mesh.
You also can make graffitis from projectors, check the lamp shadows from Odessa and just change the texture and angle.