if you are using tf3d the start menu background is plain grey. using manila(tf3d) editing, you can edit this background to any plain color. in the editor the background shows up as a 1x1 pixel, therefore it just repeats itself to fill the screen up. check this link for info on how to edit manila(tf3d)
http://forum.ppcgeeks.com/showthread.php?t=88077
if you are using classic windows today screen, then i am not sure how you go about changing the background picture