Yes, you can have your profile picture as a round image instead of a square.
You would need to edit the photo before loading it to the Black Pearl Mail app. The optimal image size for a user's picture is 300x300px.
If you are using a Mac, there's an easy way to do that using tools in the Preview app. Select Markup Toolbar:
On the left side of the toolbar, click on the drop-down for Selection Tools and click Elliptical Selection.
While holding the Shift key down, click and drag a circle around your picture. You don't have to make it perfect as once you let go, you can drag the selection around and reshape it by holding down the Shift key and clicking and dragging one of the corners. Holding the Shift key while making or editing the selection will retain the circular shape.
If the image is not already in a PNG format, you will be asked to convert it to one.
You now have a profile picture in a circular format that you can load into the Black Pearl Mail app.