Find an image you would like to have as your css console background/wallpaper and make sure its the same as your desktop or in-game screen resolution (e.g. 1920 x 1080)
Import your image into VTFEdit using the import button in the file menu. (It may take a minute to load)
Once loaded go to -"File" then -"Save As" and save it to "X:\steam\steamapps\youraccountname\counter-strike source\cstrike\materials\console" (if there is no "console" folder in materials make one)
Name the vtf file "background01.vtf" for 4:3 aspect ratio or "background01_widescreen.vtf" for 16:9
Open steam and click play Counterstrike Source to see your new background!
{optional} To remove the Counterstrike Source" text.. place this file in "X:\steam\steamapps\youraccountname\counter-strike source\cstrike\resource" folder