May be you can give your widget-css-style a background-style with url of photo from header background. But I'm not shure you must correct the width?
Best way is to search and control url, width, css-styles with firebug. Its great for diagnostic of themes. You can get a german youtoube-video. You can see the steps:
"Hochgeladen von goldeneye14782 bin 2011.11.19
So verwenden Sie Firebug auf Ihren WordPress Site des CSS (Video Tutorial) Ändern - Part1 tipsandtricks-hq.com"
Its not my own, but I trained with this video to understand deeper Magazin Basic.
Perhaps I could help you?