Clickable header – How to adjust?

My page: infobusiness3x.de

I figured out how to make the header clickable. Now I want only a certain area to be clickable. How do I do this?

code I’ve used:

.site-title a {
background: url(‘headerpicture.png’) 0 0 no-repeat;
width: 500px;
height: 200px;
text-indent: -9999px;
}

I’m a newbie, but there is no way how I could adjust the area I need with only two coordinates!?