Updating Main Property Image
The picture you see of your property on your properties page and within your admin is the Main Property Image. The system doesn't take this image from anywhere it is the image you upload when you add your property to the website.
If you would like to change this image you will need to go to Properties > Select the property > Edit. You will then see 'Main Property Image' with the option to chose a file from your computer.
Once you have done this click 'Update Property'.