If you’re using the JoomlaWorks AllVideos plugin on your Joomla site, you may want to know that as of version 2.5 (current version is 3.3 as of this writing) you CAN assign a poster image to your video. This is an undocumented feature as far as I can tell, and I only found the solution from this forum comment.
Place a .jpg image in the same folder as your FLV file. So if your video is myvideo.flv, post a matching file in the same directory as myvideo.jpg
To see this effect in action, watch the image preview load for the video posted on this page: http://www.hiltonstayconnected.com