No command to turn off the time/date, however there is a way to simulate what is needed on the phone.
1 select the pix you want as the wall paper
2 now select the _same_ pix as the screen saver
3 turn greeting message off
4 reduce screen saver time to the minimum (5 sec)
5 open the phone. wait 5 seconds. you have your pix without any overlay text
6 you may want to increase the lcd dimming time or turn it off...
this way I get to see the time/date for a while before seeing my pix without the time/date.
Hope this helps ...