You may have noticed that at times when you share a link with an image on Facebook, the image gets pushed to the left hand side of the link and gets cropped, similar to this:
The reason for that is the size of the image. Any image that is smaller than 400 pixels x 209 pixels automatically gets resized, cropped, and pushed to the left hand side of the link. If you include an image with dimensions of 400 pixels x 209 pixels or more, it will be placed below the link, like this:
Facebook actually recommends the image you include be 1200 pixels x 627 pixels, but that's not always possible if you don't have a high definition image to share. So your goal should always be to share an image with the largest dimensions possible in order to get the best quality on your post.
And while this may seem backwards, if you want your image to look the best it can on mobile devices, the dimensions need to be a minimum of 560 pixels x 292 pixels, otherwise Facebook will resize the image to 100 pixels x 100 pixels automatically.