If you have a 16:10 widescreen resolution, open the widescreen folder.
If you have a 16:9 widescreen resolution, open the widescreen2 folder.
If you don't have a widescreen resolution, open the normal folder.

In there, there is a background1.vtf/background1_widescreen.vtf file copy that to:
*whereever your steam folder is located* Steam\steamapps\*your steam name*\half-life 2 deathmatch\hl2mp\materials\console

Tested on 1280x1024 and 1680x1050, if you have any problems with it, write it in the comments.