FFMPEG QUESTION :How do I take one frame and make a jpeg with FFMPEG?

[goheadtry]

I am using ffmpeg by command line but I can't figure out how to make a jpeg screenshot can some one tell me how to make a jpeg screenshot please it would be greatly appreciated.
Maybe you can post the command.

output jpg as extesion it also needs to be 21 seconds into the video.
Just a sing frame thumbnail

I am using it in a script I am writing but it should be a basic command I just can't figure it out please help thanks.

[45tripp]

ffmpeg -i input.avi -ss 0:00:21.000 -sameq -vframes 1 frame%d.jpg

thumbnail?
you can add resize

-s 144x80