Dissolving photos on the same slide

Someone asked me if it was possible to have pictures dissolve into each other on click.

Pictures have the same size, two possibilities:

1. stack all pictures on top of each other and apply an entrance animation like dissolve or fade in.
2. instead of an entrance animation, put every picture on a separate slide and use the slide transitions dissolve or fade smoothly. Speed has only three possibilities though: fast, medium and slow.

Pictures do not have the same size:

1. apply two animations to the pictures, the first an entrance animation (fade) on click, the second an exit animation (fade) formatted as with previous. In the custom animation task pane drag the exit effect of the picture under the entrance effect of the next picture.
2. Instead of an entrance animation, put every picture on a separate slide and use the slide transitions dissolve or fade smoothly. Speed has only three possibilities though: fast, medium and slow.