Help needed for a triggered "slideshow"

Hi there,
first post here, my name is niccolo gallio and I'm writing from Berlin at the moment..

I'm trying to develop a system for a user: he needs to play a series of photos as a sort of slideshow in which he triggers each next photo to be displayed. Each next photo (which is quite similar to the one before) should be crossfaded with the old one over some time (~5 seconds). This would be fairly simple, using a couple of qt.movie objects and a jit.xfade object, where at the end of each fade a new picture is loaded in the faded-out player: I've seen this done quite elegantly searching these forums.
My "problem" is that the user needs to be able to go to a next picture even if in the middle of a crossfade and he wants to control the amount of image remaining on the matrix for each picture he triggers.
In other words I'm trying to have a system where each image added kind of superimposes onto the one before and the user can determine the permanency of each image.

Does the above make sense?

Thanks a lot for taking the time to read this

Ah, Happy new Year!