This image is used in common.css file the div.header class. But the CSS files are compiled from LESS files and are minimized, so very hard for editing.
The best way to edit the skins is to download the sources, edit the respective LESS file and then compile it to CSS file.
I am using the clessc compiler, but (maybe) any other will do the job as well.
In the downloaded source, there is a script /www/templates/build_styles.sh that compiles all the LESS files into CSS.
i deleted "url("special:skin/_images/yellow_back.png")" from common.css and fixed.
Well, this works, but if you are intended to make serious skin edits, you will need the sources and some more or less regular development process, able to merge with the future changes.