Position your timeline cusor where you wish to grab a still frame. Look in the top right of the Preview Window and you will see two icons: a save frame to clipboard icon and a save frame to file icon. Click the save frame to a file icon. Drag the saved still frame into your timeline and stretch to the desired link.
I used to use the "save frame" option, but I also like to use a velocity envelope, setting the velocity to zero at the point where I want the freeze frame.
I find the "save frame" i- as already suggested - s the quickest way to do it. You can then insert it anywhere in the video clip you like - beginning, middle or end.
I prefer velocity zeroed over "taking a picture." There is a noticeable drop in quality when you do the picture. Also, if you want to adjust the "still," you can ALT+drag on a zero-velocity clip and "change" the frame.