In order to change the property logo or any other image in an email template you will have to click on Edit Template:




Then you can select the image, right-click it and choose Insert/edit image:



This box will show up, where you can paste a link to the new logo (which will have to be uploaded to some image host website of your preference, for example, https://imgur.com/upload - upload an image and then copy image address) and adjust its dimensions:




After that, all you need to do is click ok and then save the template with the new logo.