You have created an HTML template, testing to one recipient works sending to multiple fails with message Error 152 No Server specified in URL
Your HTML Template on a standard installation would be stored in the "HTML Templates" folder any reference to src= would be the default "images" folder
If the image is not present then the URL is going to be incorrect hence the error message, go back and check your code, if you want us to check it then please raise a ticket by sending an email to firstname.lastname@example.org with the code and the RDP details to your server.
If you still have the issue or this does not help then please let us know by sending in a support request to the above address explaining your loging location i.e. http://mysebsite.com the user name you were logged in as and the location of the template you were using that gave the error message.