overlapping fade-in and fade-out

I’m using either filters.alpha(opacitiy) or style.mozopacity for fading in and out of pictures.
I’ve defined 2 square canvasses on the same place with position:absolute. The reason for this is that I want to start fading in when the last picture has not yet completely faded out.
This fading in and out works fine, as long as it doesn’t overlap in time (so fading in of next picture only starts when the last is faded out to zero), or if it does, only when both pictures are overlapping (say both are landscape). When one is landscape and the other portrait, it works fine for the overlapping part, but the non-overlapping parts of the picture that is fading out, doesn’t fade at all anymore.
NN and IE behave the same.
What can be the explanation of this, and how do I get around it?

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