When you open Pholody's photo slideshow editor, by default, the Video Window is one whole piece, so you can only have one picture on screen at a particular time spot. However, you can optionally split it into multiple cells, then let a picture take one cell other than the whole screen. By doing this you can make a photo collage, or choose a different aspect ratio for a picture if you don’t want it to be trimmed.
This can be achieved by adding Layout Points on timeline. This blog explains how to add such points.
When double-click on any blank area in Timeline Panel, if the clicked time spot does not fall into animating zone of any allocated pictures, the Video Window will be highlighted, and you can see a 
If you select a pattern other than the first one (which is the default layout), the Video Window will be split into several small cells, and the first cell will be automatically highlighted. You can then add and edit pictures in that cell, as the following picture shows.
Obviously, you can also select other cells and add pictures for them. Then you can make a photo collage like below.
The new layout begins to take effect from the time spot where you added the Layout Point, and lose its dominance when you add another one at somewhere after that spot. The following figure illustrates the concept.
Below is an example that extensively used layout options to create dynamic photo slideshow. Try to play it and see the excitement.
For any question or advice, please feel free to leave a message here.
Happy photo slideshow!