Currently Being ModeratedNov 26, 2012 10:50 AM (in response to marianneword)
The easiest way is to do it is a screen shot with the shift-command-3 keys. You will need a least a simple graphics program like preview which comes with the Mac software to open the shot, crop it and save. I learned to do it this way as iDVD would not allow copy or save of a frame.
You can also try the iDVD support which may have some better answers for you.