You need to publish your banner.htm file to a web server accessible to Internet. It is much easier to add the banner as an Image to your dashboard. This way you avoid having to publish the html file to a web service.
In addition to Ramon's suggestion, you might want to check out this knowledge base article on how to publish documents/files to the Tableau Server: Accessing the Web Server in Tableau Server | Tableau Software
Link that you have sent is helpful but I don't know How can I put html file in httpd because I don't have access to Tableau Server Machine.
I don't know if I'm understanding well your scenario. It seems you have set the size of your dashboard as Automatic to fit the size of the screen.
If that is the case, try to set the image object as Fit Image. To do this click on menu symbol (to-right) of the image object and select Fit Image.
I am doing same thing but it not worked in my case. Sometimes image is too large to fit into the dashboard and sometimes it small some of the space is blank on dashboard.