While there are lots of reasons for this error you can fix the mod_security error by adding this to your .htaccess file. [via http://thethemeblog.com] If you are using authentication then the problem is more likely elsewhere. See "Password protect Wordpress (but still use the Flash Uploader)" instead. #BEGIN Image Upload HTTP Error Fix <IfModule mod_security.c> <Files async-upload.php> SecFilterEngine Off SecFilterScanPOST Off </Files> </IfModule> <IfModule security_module> <Files async-upload.php> SecFilterEngine Off SecFilterScanPOST Off </Files> </IfModule> <IfModule security2_module> <Files async-upload.php> SecFilterEngine Off SecFilterScanPOST Off </Files> </IfModule> #END Image Upload HTTP Error Fix