You can choose between 3 types of shapes for the image that will appear in your reviews and orders type notifications:
1) Create a new notification or edit an existing one.
2) Go the second step ("Placement") and scroll down the page.
3) Under the "Content" section, go to "Image shape" and select your desired image shape using the dropdown.
4) Save your changes using the "Launch" button at the bottom of step3 ("Design").