All of your photos have an aspect ratio which is the relationship between the height and width of the image. If photos aren't square then they can be tall and narrow or short and wide. The aspect ratio determines what looks good in the area where you drag and drop your photos.
If a wide photo is place in a tall image placement area only the centre of the image will show. You can move your photo around inside the box using the photo editing tools but if the aspect ratio of the image is different to the area in which you are placing it, it will not fit to display the full image.
The same thing will occur is your placement area is wide but the image is tall. The top and bottom of the image will not be displayed. This can be adjusted but not completely resolved so that the whole image is displayed.
You can use the crop/position tool to move the photo around inside the space to better display the parts that you want to show, or you can change your layout selection to fit the aspect ratio of the photo.