My themes that include a banner come with two of them. The first has the theme's name on it in stylized text that matches the theme. And the second is a text free banner that you can add your own text to. Upon installing my theme the banner with the stylized text is the one you'll see. The next three posts show you where to find the text free banner and how to upload a new one.
WHERE TO FIND THE TEXT FREE BANNER
Go to Admin > Themes > Theme Manager
Hover over the theme in question and click on the Images button.
Scroll to the bottom section named Custom Theme Images. Here you'll find the text free banner. If you want to add the text yourself click on the image. You'll see a window pops up with the banner at actual size. Click and drag / copy and paste the banner to your desktop. Now you can add your own text.