If your video is an .swf, then create a new Flash file with your still image in
the first frame, and a stop() in that frame, so it won't play until you hit
your play button. On the second frame (after hitting the button) have your .swf
load. Your .swf will play through, then loop back to the beginning where it
will stop again with your still image.
If you are working with an .flv, then you should go back to your original
video file and make the edits there.
If all you have is an .flv, then you can still add overlays to the video
within CS3 to hide your unfortunate ending. For details on this, either do a
search for CS3 video editing, or get a good book (e.g. the Friends of Ed book
on CS3 Video) or the Colin Smith video tutorial, Flash CS3 for Designers, at
lynda.com.