In Resource/ui go to mainmenu.res and find "Background". There's a bunch of if_ values there for the background image, but I think if you just chagne all of them to the same image it'll work fine.
Take the image you have and make into a vtf and save it as it "background_2fort.vtf" and make another called "background_2fort_widescreen.vtf". Then in mainmenuoverride.res have the image in the background section be background_2fort.vtf or background_2fort_widescreen depending on your aspect ratio.
Use this thread next time: https://www.teamfortress.tv/19073/hud-editing-short-questions-quick-answers