In xine, you can take video screenshots by hitting the 't' key while the video is playing. This will save the current video frame as a jpeg in your home directory. If you use mplayer, you'll need to enable the screenshot filter first. Just add 'vf-add=screenshot' (without quotes) to ~/.mplayer/config and then hit the 's' key to save frames as jpegs. If you want to take multiple screenshots that 'sample' a video file, try this:
$ mplayer -ao null -vo jpeg -frames 10 -sstep 20 foo.avi
This will take out ten frames, one every 20 seconds, and save them as jpeg files.