In order to change the property logo or any other image in an email template you will have to Access Property Management / Email & Printouts and 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, the image was uploaded to https://imgur.com, then we copied the image Direct Link and adjusted its dimensions:
After that, all you need to do is click ok and then save the template with the new logo.