This option specifies the image upload extensions which you don't wish to use in FCKeditor. Example:
FCKConfig.ImageUploadDeniedExtensions = ".(bmp|gif)$" ;
If you leave this option empty none extension will be denied.
NOTE: if you leave ImageUploadAllowedExtensions empty and you will put something in ImageUploadDeniedExtensions e.g.bmp it will allow the upload of all the files exept the bmp. However it isn't a good way to secure your server from unwanted uploads. The best way is to put all of the prefered extensions in ImageUploadAllowedExtensions and put the unwanted ones in the ImageUploadDeniedExtensions. Only in this way you will secure your server from junk.