I have been playing around with this.
'Settings', 'General', 'Passcode Lock' -> set it to 'On', 'Picture Frame' -> 'Off'
Startup Screen -> No picture frame but you have to type the passcode when you turn the iPad on
'Settings', 'General', 'Passcode Lock' -> set it to 'Off', leave Picture Frame 'Off'
Startup Screen -> Picture Frame returns even though it's set to 'Off'
Googled this and it seems to be a 'feature'.....
Tim