Change Twenty Thirteen Header Image Size

Hello:

In spite of all the information I’ve found in reading through several dozen posts on the subject I’m still having changing the header image size on the Twenty Thirteen theme.

First, the obvious bits:
– Yes, I’m using a child theme.
– Twenty Thirteen version is 1.2
– WordPress version is 3.9.1

What I’m trying to accomplish:
I need header image dimensions of 1100px wide x 275px tall. From all the other posts I actually have a display image area in the desired dimension and the cropping feature is not complaining, that I can tell.

Problem:
Using the Custom Header dashboard I upload an image sized to 1100 x 275. But the image appears to get sized UP to 1600 wide, then a section of the scaled up image in the dimension of 1100 x 275 is displayed. So I end up with a piece of the intended image displaying in the correctly sized block.

I’ve looked for quite a while but I haven’t found any other posts that describe quite this problem.

Any thoughts?

Thanks for your help.