How do I stop Photos from cropping all my photos in a slideshow?

When you use the Classic theme, click the options on the sidebar. There is an option to "Scale the photos to fit the screen". If you enable this, the photos will be scaled to fit on the screen without cropping.

The more elaborate themes are useless, if our photos have not been hand-picked to fit the elongated frames of the fancy templates. Photos is using a fixed sequence of templates, and we have to provide the photos in an aspect ratio, that will work with these templates.

Hi -

It depends which theme you are using. Some of them are set for a widescreen format and crop the image to fit. I think two which don't crop are classic, and vintage prints.