3.7.11 works fine on a different site, but this is on a new website so I may be missing some setting somewhere.

Yes, I have added this to the HTML/CSS Inserts section:

div.header-image-container img {width: 100%;}

and/or

div#imagecontainer {background-size: 100% !important;}

And yes, I have tried all the browser settings in "Style & Configure Layout"

Anyone with ideas?

What is the URL?

http://coloradoscenictour.com/

It may or may not go without saying, but I'm concerned about IE8 because my client is still using XP.
I wonder if a test of IE8 will give the same results with a different OS?

1) why are you using a GIF?
2) remove the space from the file name

The default file type for "Save for Web" in Adobe Illustrator is .gif, and I assumed they must be smarter than me.

A .jpg file with no spaces in the filename does not solve the problem.

I'm curious about finding two different names for the header image container in the fixes lmilesw came up with:

div#imagecontainer {background-size: 100% !important;} - Mar 19, 2012, 10:00 AM

div.header-image-container img {width: 100%;} - Aug 30, 2010, 02:24 PM

Is that area now possibly identified as something else?

By the way, sorry about our misunderstanding (from two years ago!! - I just checked my messages yesterday). Having just made a somewhat feeble offer, it certainly appeared to me that I had gotten a giant "Drop dead" in response.

I'm curious about finding two different names for the header image container in the fixes lmilesw came up with:

div#imagecontainer {background-size: 100% !important;} - Mar 19, 2012, 10:00 AM

div.header-image-container img {width: 100%;} - Aug 30, 2010, 02:24 PM

Is that area now possibly identified as something else?

They both reference the same thing, one uses the 'ID' of the <div...>, the other references the 'CLASS' of the <div...>

You will want to replace the %image with

<div id="my-image">
    <img src='http://yourdomain.com/wp-content/path to image/imagename.ext' />
</div>

using the appropriate path and file name then add to the CSS Inserts option

div#my-image img {height: 100%; width: 100%;}

note: you will not be able to use the other header image options since you have 'rolled your own' header image at this point.