Why not use this picture for images/thumbs/thumb_flv.png instead of the current movie unknown picture.
http://wwwimages.adobe.com/www.adobe.com/downloadcenter/images/flash/flash_128.jpg
FLV files aren't supported by Coppermine out of the box. However, if you want to use that file for flv files, it should be enough to upload it as images/thumbs/thumb_flv.png to your gallery.