Instructions
Download a "spray" image from a "Team Fortress 2" website or create your own. If creating your own image, use an image size of 256x256 and save the file as a TGA file. Convert the TGA file to a VTF file using a program, such as VTFEdit.
Go to the following folder in your "Team Fortress 2" directory: /Program Files/Steam/steamapps/(your steam username)/Team Fortress 2/tf/materials/vgui/logos/
Place the "spray" image file in the logos folder. If the logos folder is not in the "vgui" folder, then create it.
Launch "Team Fortress 2."
Go to "Options" and click on the "Multiplayer" tab.
Browse to the folder location where you saved the image.
Click on the image and click "Open." You should see your "spray" displayed.
Exit the options menu and start a game.
Press the "t" key on your keyboard and the image you chose will be spray painted on the wall or floor in the game.