Below is an image we're starting off with.
Copy the image to your C:\Program Files\Steam\SteamApps\
The image should now be imported and visible on the preview window.
The nice feature about this import utility is you can add as many spray logos and they will appear on the drop-down menu for use later on. Here is what our final spray looks like in-game.
Cool huh? Thats about all there is to creating spray logos. Once you get the hang of creating them you'll probably want to customize them futher by adding animation and transparency amongst other things. Hope this helps you make spray logos with easy.