I don't see the default header image. I see a homemade header on your blog and I'm using Firefox 20.
If you are not running one of the browsers and versions listed at http://browsehappy.com/ please try upgrading or switching first.
Note: If you are using an IE9 browser in compatibility mode that makes it render as IE7 and IE7 is not supported. To disable see > http://www.sevenforums.com/tutorials/1196-internet-explorer-compatibility-view-turn-off.html
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1. Try clearing your browser’s cache:
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3. Try with all browser extensions or add-ons temporarily disabled.
4. Try enabling HTTPS: http://en.support.wordpress.com/https/
If it’s still happening, let us know which exactly browser (and version of it) you’re using by checking here if necessary http://www.whatbrowser.org/ and also post any error reports you get.