This is the thing.
When you put code in the banner code input that code is inserted "as is" in Chevereto. So whatever you put in the banner box is placed directly in the script. So, the code is inserted directly with absolute no modification. This means that if the code doesn't come with basic stuff like the width of the banner or anything like that it will be displayed full block (in other words all the "row").
So, like I said... The problem is that the banner doesn't come with a self box container so either you add it or you live with it. This has nothing to do with Tech support, the problem is that the banner code doesn't wrap himself in a container.