From iOS 5.0 Apple made a change to have the apps enter the background when the screen is turned off - and then close down after a while.
That is why I added the Backpack Mode feature. ..but yeah, that won't help you with the slideshows.
So far all I can say is go into the Settings app and turn the screen brightness all the way down. ..still not good enough, but you'll be using less battery.
I'm working on something that may be another solution, but that won't work until a newer version comes out (and that's not in the version currently in review).
I'll write back when I have more info for you..
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