need to find a way to insert html in top banner of main page

[gjh42]

I really don't have any idea what your banner code should look like. I can only advise you about code placement within the file. There is a header banner built into Zen Cart - the display code is in the branding display section of tpl_header.php. You should be able to either use or modify that to suit your needs.
You might check the FAQ/tutorials for information on banner use.

If you want the banner to go in the open space between your logo and tagline, be aware that your template has a fluid layout and will expand and contract to fit the window. If the window is small, a banner in that location may cause overlapping or other undesirable effects.

You may be able to use min-width: on #mainWrapper to ensure that the page doesn't try to shrink beyond a reasonable size.