Cropped photos in slideshow.

I think it really depends on the theme used.  However, for those themes that present the image in full frame, you should use 16:9 cropped images since the output is going to be 720 or 1080 (HD) ratio.  Regular 3:2 photo ratio or even 4:3 should work for most  photos but there will be black bars at the sides.

Other themes that present the full image within a template background generally do not crop a photo of any aspect ratio or of any orientation.

Yet other themes animate, and therefore crop, images in a variety of ways.

If you want more control over your slideshows, you need to use a different editor such as Premiere Elements which will give you full control over whether or not an image is cropped.

I have Photoshop Elements 2025 with same cropping issue in the slideshows produced by Organizer. The Greg S. reply says that the image should be "cropped to fill the frame". What frame? Specified whare? Does it vary based on the Slideshow Theme? The slideshow editor imports the selected images and does not provide any feedback about acceptable frames or cropped frames. I have seen it crop portrait and landscape images. I am looking for an answer that specifies the "frame" in terms of perspectives and aspect ratios. In other words, what are the unspecified constraints that we need to use for frames in terms of perspectives, aspect ratios, and resolutions? If these are specified within Elements that information is well hidden.

It is the same story for square images.  They have to be cropped.  Otherwise, they will have black bars to fill the non-square frames.  The size of the file (other than the pixel dimensions and aspect ratio) has nothing to do with it.  If you want to show us a screenshot demonstrating the cropping, I may have a better explanation for you.

Are the photos portrait rather than landscape orientation?  If so, the photos have to be cropped to fill the frame.  Otherwise, you will have black bars at the sides of the images.