Every photo has an aspect ratio. This is the relationship of the height and the width of the photo. While the actual ratio isn't critical, what is important is that, unless they are square, photos can be tall and narrow or short and wide. How tall/narrow or short/wide they are will determine what will show in the image well (the area into which you drag and drop your photos).
For example, if the photo below (which is a wide photo) is put into a tall image well, only the center part of the image will show. You can use the photo editing tools to move the photo around inside the box, but not all of the photo can fit into the box width-wise because the box is too narrow.
Conversely, if the box is too wide, then not all of the image will be able to fit vertically because the photo has to expand to fit the width of the box. Again, the photo can be adjusted up or down, if only part of the photo is needed, but to show the whole photo, the image well or project (if setting the image as a background) has to be the same proportion as the original image or part of the image won't fit.
Tall image well:
Image well too wide for the photo:
If you want to adjust the photo, just click on the blue boxes at the corners or edges of the page and you can make the image well wider or narrower, taller or shorter so that it better fits the image you want in that place. You may need to move other images around on the page to make the layout work for you.