Hello Markus Berchtold,
Unfortunatetly I did not succeed in reproducing your issue. I tested 4.1.1339 (official release) and an upgrade from 4.1.1339 to 4.1.1395 (official release to service pack 1) on several browsers - firefox 4.0, chrome 11, ie 7 and ie 8 but all my tests displayed nice images.
What version of Sitefinity CMS do you use? Or what kind of upgrade? Is your issue reproducible on all browsers or only on a specific one?
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