You may see this error if your image fails to load inside the editor:
In this case please copy the image URL and open it in a new tab in your browser. If it loads correctly in a new tab, that means that the browser is blocking MailDeveloper from opening that image inside the editor, which is a security measure in some browsers. It is caused by the fact that MailDeveloper is loaded under a secure https-connection, and some browsers don't allow loading insecure http-based images in that case.
In this situation the image should still work fine in the exported email; it is simply not visible when editing the email in MailDeveloper.