If the Property Feed toggle is switched off for a panel, you can change the image chosen for any property.*
Click the camera icon found in the middle of the image:
Then you'll see all the images connected to the property.
Click on any image to update your email template.
* It's important to be aware that if the property feed toggle is switched back on - making the panel dynamic, this will update regularly and the image will revert to the first image supplied from your property feed. If you are using dynamic panels and would like to show an image that is not the first on your property feed, we recommend changing the order of the images on your feed, so that your preferred image is first.