Hello Andreas Lohr,
Thank you for contacting us.
There is a known issue when trying to use animated images in RadPanelBar. This issue have been solved in the successor to the obsolete RadPanelBar, namely RadPageView. Thus, I would recommend that you upgrade your application to the latest version of our controls - Q2 2010 SP1 (v2010.2.10.806).
I hope you find this information useful. If there is anything else I can assist you with, do not hesitate to contact me.
the Telerik team
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