If you’d LOOK at the page source of the link you posted, you’ll see the style information for the header image is right there. So, go into the theme editor and edit the header.php (if the code is not there, it’s in one of the other template files, I don’t use the default so I’m not sure). There, you’ll be able to adjust the image and do whatever you want. As was mentioned earlier, you should look for a better theme to customize, as the default is difficult.
I’d also suggest you read some basic tutorials on CSS, HTML and images, as that will be far more fruitful for you than your current method of problem solving. How to do what you want has been clearly explained, you need to help yourself a bit as well, I’m sure you can understand how frustrating it can be to re-post the same solution over and over again.
“Seek and YOU shall find!”
Good luck,
Michael.