Yes, you can replace both the thumbnail and preview with your own version. To do so, navigate to the file, and go to File > Edit. Then you can use the "Browse" button to select a new file for the thumbnail and/or preview. For more information, please refer to this support article.
All major picture formats are accepted.
It is also possible to change the way thumbnails are shown without cropping them, click here to read more about thumbnail display mode.