[videoblogging] How can I make a .jpeg "frame" to illustrate what video is about?

[Ed]

Hi, I use window movie maker to prepare videos to post on youtube and
other such sites.  When I upload the videos, certain sites ask me to
give them a jpeg "frame" to illustrate what the video is about.  I
would like to create a frame independent of the video that I could
offer the site, or take a frame out of the finished video to use for
this purpose.  Can someone please tell me how to create a jpeg with
text on it for this purpose and also to take a frame out of the movie
file and make a jpeg out of it.  OK, thanks in advance for all your
help. Ed.