mplayer -vf screenshot tv:// driver=v4l2:width=800:height=600:device=/dev/video0 -fps 15 

# and that gave me 800x600 video, and after I pressed 's' i obtained 800x600 in the snapshot.
# q to quit

# I did the same for 1024x768, 1280x1024, and 1600x1200 and obtained a 1024x768, 
# 1280x1024, and 1600x1200 video and snapshot respectively.

# or get shots from a video
# mplayer -vf screenshot some.avi

# use keys
# 1 and 2 - Adjust contrast.
# 3 and 4 - Adjust brightness.
# 5 and 6 - Adjust hue.
# 7 and 8 - Adjust saturation.