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I would guess that flicking into sleep mode and waiting for the final cover or image to be displayed would be long enough. Common sense would pretty much dictate that you'd power down everything else and then display the image last just prior to sleeping the microcontroller -unless something in the hardware design forces you to do otherwise. But then, common sense is not nearly as common as we'd like to think these days...
Samhy,
I think I understand the point you're making regarding multi-tasking/threading and multiple operations(and I'm not sure of just how much of that might be an issue), but any properly designed system like that has to be designed to terminate or put on hold those tasks if a sleep mode is entered, or through a semaphore or signaling device indicate that sleep mode must be postponed until the tasks or threads can complete.
Basically, the sleep mode code might signal that it wants to sleep the unit, and usually wait for an indication that it was okay to do so from the task scheduler after it had shutdown scheduling operations after ensuring that any time critical processes had been informed and shut down or completed, again with semaphores, messaging or at least flags of some sort.
I'm of the mind that a properly designed, programmed and engineered device does not indicate it is in a low power or sleep mode state if it is indeed drawing excessive current beyond the normal expectation for that mode. If it can't shut something down then it is either broken, poorly designed, or should not indicate it is in sleep mode -or possibly more than one of the former is correct. That is not to say that said behavior cannot be fixed or improved.
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