Images not included in eMails

Dear Support team,

I purchased your eMail designer yesterday and started working with it. I set up a first email template in my staging environment. The test email did not provide any images, neither in Outlook (desktop), nor in my mobile eMail programm (iphone) nor in my alternative Gmail address. I am sending you screenshots of what it should look like (WordPress backend) and how it came out in all three eMail clients (Outlook, iphone, Gmail). I am assuming some kind of broken connection between the plugin and my media library. I would very much appreciate if you could help me with this so I can install the plugin on my live site once I get it to work properly in my staging.

Thank you very much and kind regards
Katja