From the map, click on any vessel, lighthouse or port.

From the details panel, tap on the photo that appears

From the photo gallery, when on your preferred photo, click on the stars located at the bottom of the screen.

A new screen will appear at the bottom of the interface and you can rate the selected photo on a scale from 1 - 5 stars.

Once you tap on a star to rate the photo, your rating will be saved. Please be aware that it may take some time to update ratings counts.