To change the main menu's background you need:

• VTFEdit[nemesis.thewavelength.net]
  
• A picture set at the resolution of what you run your game in or at the same aspect ratio. I play Synergy in 1920x1080 so I need a 1920x1080 picture. You can find your resolution in this menu shown in the picture below.

If you have everything you need, open VTFEdit. Click on "File" and then on "Import" and find your picture.
When you opened your picture, set all setting the same as shown in these pictures below. Otherwise you get ♥♥♥♥♥♥ up looking image.
Then go to "File" and then "Save as" and save it as "background01" and save it again as "background01_widescreen" without the ". You don't need "background01_widescreen" if you play with a 4:3 resolution.

Now place your .vtf file in:
Steam/steamapps/common/Synergy/synergy/custom/my_custom_stuff/materials/console
You need to make the last 3 folders yourself.

If you followed all steps correctly your game should look like this:

P.S.
Only the startup loadingscreen is still default. The main menu and the other loading screens are changed.
And If you see any grammar mistakes, please tell me.

http://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=441532280