Not all of image showing in Slideshow

I'm a newbie and not very technically oriented. I imported a portrait format image from the www and in Slideshow it's only showing about 2/3 of the image even though I set it to view on a computer screen (vs iPhone or other). Is it that the image is too large to begin with? This also happened with the next slide that is in landscape format; I put text in the upper left, but the Slideshow is not showing it either--just the bottom 2/3. What do I do?