You would need to edit the photo before loading it to the Black Pearl Mail app.
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 you can load into the Black Pearl Mail app.