Color section height = image height

Hi, guys!
It’s my first post here, so first of all, I would like to thank You all for a great theme!!!

How can i make the height of the color section be equal to the height of the image, selected as scrollable background?

Example:

For color section background (that will be scrolled with the page) i use an image of 1000x400px.
The browser window is now 1000×900, so the image is stretched to its full width and its height is 400.
The height of the color section in this case should be also 400px.

Then I make the browser window narrower – 700px, leaving its height as it was – 900px.
The background image is proportionally resized to 700×280.
The size of the color section should also proportionally change to 700×280!

This is of course usable when there is no content in color section.

How can it be done?
I’m ok changing php or css, so just tell me what to do, if its possible! )

Thanks in advance!
Andy