To change the main image from the existing one to
another that has been uploaded, log into the control site, go to the property
page and scroll down to the Photographs section:
Click on the image that you wish to
make the main image:
Click on the arrow next to the pencil icon at
the top right of the screen (shown above), then click on 'Yes' for Main picture
and click the tick icon.
Click the ‘X’ at the top-right of the screen to
go back to the property page: