After logging into the application with your phone, select the hamburger menu on the upper left.

Select the icon next to your name

You will be given the option to either take a photo with your device, or to include an existing  image.

To take a photo, select Take Photo.

Follow the instructions on your device, and then if you are satisfied with the image, select Done on the lower right.

To select an existing photo from your device, select Choose Photo

To delete your photo, select Delete Photo 

You will be asked whether you wish to delete the profile image.  If you agree, select Yes.