Drag the picture to move it. Scroll over it to zoom. The whole frame is exported (stretched sideways into the square — the game squeezes it back); the lines mark where the Visual and Misc tabs cut it off, and the shaded part is never shown in-game.
Menu preview 770 × 420
Drawn at the real menu size from the screenshots, 1:1 pixels when there is room. The game puts nothing between the picture and the text — no dimming, no shadow — so if a label vanishes here it vanishes in-game. Switch tabs: the taller Misc tab simply reveals more of the picture below.
Put raw.data in your tf folder, then enable Visual → Custom GUI background. File is always 4,194,304 bytes. The PNG looks stretched sideways — that is the real file; the game squeezes it back.