On the bottom of your theme is a MistyLook link and when clicked it connects to the theme description. The header dimensions are The header image is 780 by 189. It's also listed here http://en.support.wordpress.com/themes/custom-header-image/
If you are not running one of the browsers and versions listed at http://browsehappy.com/ please try upgrading or switching first.
If you are, please do these:
(1) Try clearing your browser’s cache http://www.google.com/support/accounts/bin/answer.py?hl=en&answer=32050
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
(2) Make sure that you have third-party cookies enabled http://en.support.wordpress.com/third-party-cookies/
(3) Try with all browser extensions or add-ons temporarily disabled.
(4) Try enabling HTTPS: http://en.support.wordpress.com/https/
If you aren't sucessful, let us know which browser (and version of it) you’re using and post any error reports you get.