Installing Sprays - Import
Go to Options in-game and import a TGA/BMP etc from there.
Half-life: Deathmatch
The .vmt and .vtf files go into this directory:
_YOUR_STEAM_USERNAME_\half-life 2 deathmatch\hl2mp\materials\VGUI\logos
Counter-Strike: Source
The .vmt and .vtf files go into this directory:
_YOUR_STEAM_USERNAME_\counter-strike source\cstrike\materials\VGUI\logos
Other Mods
The .vmt and .vtf files go into this directory:
X:\Program Files\Valve\Steam\SteamApps\SourceMods\Mod_Name\materials\VGUI\logos