If you are mainly interested in making a great slideshow, you can adjust the size of your photos so that they look great in a slideshow.
A good size for an image is 900 pixels tall and 1350 wide, which is an aspect ratio of 1.5 (the same as a 4x6 print.)
A wide, short image will appear small and at the top of the screen:
Re-size the image to 1350 wide and center it on a 900 pixel tall black canvas so that it appears in the center of the screen:
Sample slideshow: https://www.dotphoto.com/go.asp?l=bdag&AID=6744464&IID=275815529&T=1&S=Y