Adding fixed position header logo in 2013 theme

I want to replace the starter background which appears to be located center with an image with text which I always want to show regardless of screen size. I want to take advantage of 2013’s responsive design so my text containing logo adjusts in size with the screen size. To do this I believe I need to change the location of the background from CENTER to LEFT.

I’ve found the CSS with Firebug…

site-header {
    background: url("https://79.170.44.91/marionandpeter.com/group1/wp-content/uploads/2014/03/cropped-Chrysanthemum2.jpg") no-repeat scroll center top / 1600px auto rgba(0, 0, 0, 0);

… but I can’t find this code in styles.css located in the twentythirteen folder. The firebug location is \marionandpeter.com\group1\#2.

Is this the right approach to place a logo inclusive site header so that it always shows ?
If so where is the css to change ?

Here is the test site showing the issue https://marionandpeter.com/group1/

Thanks