The Photo Gallery add-on will try its best to accommodate the imported images into the Google Slides canvas. Please note that there are 2 important factors that need to be taken into consideration:
1.) The image dimensions (width and height) - uniform sizes will yield the best result
2.) Page setup used in Google Slides
If the images have the same dimensions (width & height in pixels), then the images will fit snugly within the Google Slides canvas. But if the image dimensions differ, the script will attempt to output the images as best it can.
As demonstrated in the screencast below, we used same-height images. And when the Slideshow add-on finished executing, it laid out the images perfectly based on their height. From there, you can adjust the page setup to any dimension you want. We need to do this last bit since Google Slides has some limitations - but this doesn't mean we can't work around it.
Demo of the add-on using same-height images (altering the page setup later on).