Instead of creating a new cron job in your script, and having to remove the "old" cron job every time too, you should use the
at command instead. The
at command was created exactly for that, to let a command run once at a specific date/time. This way, you won't have to remove the old cron job.
So, you could have your script, lets call it "capture_from_webcam.sh", looking like that :
#schedule next capture for tomorrow + 15 minutes
echo "/path/to/capture_from_webcam.sh" | at tomorrow + 15 minutes
# capture from webcam
And to have if execute for the 1st time, at midnight on January 1st :
echo "/path/to/capture_from_webcam.sh" | at 00:00 01/01/2010
For more information, see the at man page.