Go to \Steam\steamapps\<username>\team fortress2\tf\materials
Create a folder called VGUI
Now. What you need is an image. You can use .jpg, .bmp, .tga, .vtf
Note that the acceptable image sizes in terms of length and height are: 256, 128, 92, 64, 32, 16. 8, 4, 2 and 1
So if you can stay somewhere within the maximum 256 height/length. Then you are safe.
Place the image in your newly created VGUI folder.
Start Team Fortress 2 and go to "Options"
In options, you usually have 6 tabs at the top. Click on "Multiplayer".
Now you are in the "Multiplayer" tab. You should now be able to see a button called "Import Spray..." Click on it and go to your VGUI folder. Select the image and then click "Open". If you did not get any errors about the size. Then you now have a Spray image you can use in-game.
Hop onto or start a server. Walk up to the wall and press the spray button (default is "T" )
For our members, I provide you with a Spray image: