Organise Video Clips with Poster Frames....

While you understand>>  Wiindows explorer you can view the folder with thumnails which shows the first frame of the video , This is the actual first frame as you said, to alter that you would need to insert a new image to the start of the video.
A photo as far as I know will not work.
Windows movie maker2 may help you there and or Nero Vision.
Same principal they both use a video timeline and export as feature.
You need to create a new image but in mepg or avi.
How I foudn a workaround for this
what I do is take a snapshot of a segment of a video,
 pausing the video hit print screen, and save that as jpeg.
Drop this jpeg onto media player classic and play it full screen or boxed.
Then using cam studio I capture about 10 secs of frames of the jpeg and save it as an avi.
It's now a  10 sec video.
Drop that onto the video timeline in NERO Vision or windows movie maker and then add the avi in after it.
export the new video  merged with the new thumbnail.
I also use this trick to add in chapters.
Hope you like my tricks.
Merete

I understand what you are saying and it all sounds possible but a lot of work! :-)

are there no comerical or opensource software that can do this automatically - i.e play a clip - you select a frame and it inserts it as the poster frame at the beginning? If you've got a lot of clips you dont want to be switching between apps all the time just to complete one video