Sizing of Photos - Slideshow

And just to give you an idea of what your sample photo should look like with a 16:9 crop but without black bars:

The prescence of black bars is all a matter of aspect ratios.   You can't fit a square into a rectangle of unequal sides without having them. 

A slideshow will typically have an aspect ratio of 16:9 which is what you will see on a current TV.   A typical photo will have a ratio of 3:2 or 4:3.  Your screenshot displays a photo that is in portrait orientation.  The only way you can display such a photo without black bars is to drastically crop it so that the image pixels fill the screen.  A photo that is in landscape mode will need less cropping. 

Yes, you can crop the photos to a 16:9 aspect ratio so that you have full control over what parts of the image will be displayed. You can do this in the Editor which has a crop ratio template of 16:9.  You can also do the cropping in the Organizer's Instant Fix editor. The Organizer is probably easier than the Editor because it allows you to batch edit multiple photos, crop them individually, and then save automatically in one step. 

There really is no need to use safe areas when creating a slideshow. That feature was once needed for viewing on old CRT monitors and TVs. 

And, in my experience, most family photos are improved with cropping.