First we start off with going to the old Gmod main folder. Which is mostly located in: "\Steam\SteamApps\common\GarrysMod\garrysmod\"

Here you will go into the "materials" folder or if you don't have one, make one. Now make another folder inside that folder and call it "console". Now is the tricky part.

If you don't know how to create a VMF, or VTF file. I suggest you looking it up on Google.
Here are some useful programs:
VTF/ VMF Editor: Nem's VTF Editor[nemesis.thewavelength.net]
VTF Plugin for Photoshop: Photoshop VTF plugin
VTF Plugin for Gimp: Gimp VTF plugin

First we get a picture of whatever you want. Open up the Photoshop or Gimp and create a new document. The file should be 512 x 512. Then save the file in "\Steam\SteamApps\common\GarrysMod\garrysmod\materials\console".
Save the file as a vtf and call it "background".

Now open up the VTF editor and go to Tools < Create VMT File.
The Base texture is the background file you created. Then go to the options tab and make sure Shader is set to "UnlitGeneric" then hit create. Save the file in the same place where background.vtf is and call it "Background01". Then open up the "Background01.vmt" and it should look like this:
"UnlitGeneric"
{
"$basetexture" "console/background"
"$vertexcolor" 1
"$vertexalpha" 1
"$ignorez" 1
"$no_fullbright" "1"
"$nolod" "1"
}

If it does not look like this, then add the parameters you are missing.
Then hit save and you are done. Enjoy!