Well as David said, which also confirms my suspicion, it is always looking in the .pk3 file for media, such as sounds, music and artwork.
You'll be able to move the menu around by looking at the files as i showed you and even add new parts to the menu if you like, like oticzs has.
I tried giving an absolute path to a different jpg but its still looking in the pk3, so all i got was a black square with a white border around it.
If you change the background.jpg to something else in the pk3 it will undoubtedly work, but then your client will fail, as David said, the pure tremulous checks that servers perform.
Ah well.. i tried

EDIT: Indeed changing the image does work - keeping the same file name in the pk3 and swapping it for a different image, but joining "pure temulous" gives "ERROR - invalid game folder"