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, 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.