The new crop option offers an alternative to the previous stretching display.
Let me explain how it works.. if you choose to stretch images, then every image will be stretched without regard to aspect ratio to fit in a full panel -- you can set thresholds for aspect ratio stretching limits in the other options but this just affects what sized panels are allowed.
The new cropping option will never alter the aspect ratio -- it will enlarge or reduce the image in order to make sure there are no blank areas, and then choose a random region to display to fill the panel.
In this case you can still enforce the aspect ratio of allowed panels, and you can also set a limit to how much you want to allow the program to scale down the image to fit the smallest axis.
So if you set the minimum reduction to 0% then the image will be reduced such that one edge (horiz or vert) of the image will always be fully displayed, and the other axis will be cropped randomly. If you set the minimum reduction to 100% then it will never shrink the image and will just pick a random region to display, equal to the size of the panel/tile.