Flash uploader and Flash 10
From Joomla! Documentation
There are many problems with lots of different software due to the new Flash Player v 10. Until there is a more permanent solution, it is recommended you simply turn off the Joomla! Image Uploader in the Global Configuration Manager.
Some are uninstalling Flash 10 and reinstall Flash 9. That is not recommended since Flash 10 is a security release. Even though the Flash 10 security updates do not apply to how Flash is used with Joomla!, downgrading could make your Web browsing experience more vulnerable for other applications.