Hi Florence
There is no problem in doing this,,,,
Put the video onto the timeline
Play the clip (or move the red cursor line) to the point where you want to extract the image
Make sure you are in SEQUENCE PREVIEW and you will see a "Snapshot" button under the preview pane.
Left click the button to extract your image. (Note you can also down-arrow the button to see resolution options)
Your image ( a .png) will be copied to the image bin as well as your previously selected download folder.
Note that you can set your download folder under Tools/Options/Media "Copy files to:" box
Hope this helps.
Nat