Thanks kardotim! It looks like you shifted the site title area to the left to simulate making it look centered with this CSS:
.site-header h1 {
margin-left: 15%;
}
I think a better approach would be to make a smaller image that doesn't have extra white space on the right like the one you're currently using:
http://chiropractienijmegen.files.wordpress.com/2012/12/site-header2.jpg
Then you could remove the "left" keyword from the "background" property. Here is an example, replace this:
background: url('http://chiropractienijmegen.files.wordpress.com/2012/12/site-header2.jpg') left center no-repeat;
With this, and click preview:
background: url('http://s.wordpress.org/about/images/wordpress-logo-notext-bg.png') center no-repeat;
I also noticed that using the image technique I first recommended, it won't adjust for small screen sizes like mobile devices. You can make a smaller version of the image and upload that as well to use for cases like that. Here is the CSS you would add:
@media screen and (min-width: 600px) {
.site-title {
background: url('320_PIXEL_WIDE_VERSION_OF_THE_IMAGE') center no-repeat;
}
}
Replace 320_PIXEL_WIDE_VERSION_OF_THE_IMAGE with a link to a smaller version of your logo.