Hey guy,

first I want to thank you for that great theme!

How can I add custom html instead of the picture in the header? I want to place Ads there. Where in the header.php can I do that?

Thanks

in version 3.4.x you can put widgets in the header

So long as you want it to be at the very top of your page, you can put HTML in at
ATO -> HTML/CSS inserts -> Body top

That's a great place for image maps, images that are wider than the layout container (eg. socialbootstrap.com), a clickable image with a clickable area smaller than the layout container (eg. vividmargin.com), dynamic headers, or html-based stuff.