Perhaps there is something very obvious I have missed, but I replace the background_01.vtf file or whatever with the one I want to replace it. Simple right?

Not so apparently. I start gmod and it shows the original background. While it loads it then switches to the new background, and when it is done loading goes back to the original background. Is there something in the settings I missed? I was only given a .vtf file to replace and a sound file for startup.

Solved it myself. On a widescreen monitor the game creates two .vtf files for the background. One is “background01.vtf” and the other is “background01_widescreen.vtf”. In this case I had to replace both of those files. This required me duplicating the vtf file I downloaded and renaming the second one to match the widescreen designation. I accomplished this with a simple copy and paste of the file and a renaming. Simple.

Also every single background I downloaded had the file they were telling you to simply copy and paste “backgroun01.vtf”. Every single one forgot the d, way too many to be a coincidence. Authors of backgrounds may want to either spell their damn file name properly, or you are going to want to put it in your instructions to rename the file upon download.