First, you need to download a .vtf file to your C:\Program Files\Steam folder. After that, and in-game, you go to options, then the Multiplayer tab. Click on import spray and find that file in C:\Program Files\Steam. Click apply and off you go.
isn't it you need to have both a vmt and vtf form of a spray then put it in Steam\steamapps\YOURNAME\team fortress 2\tf\materials\VGUI\logos and then put the same vmt file into your UI folder which is in your logos folder.