As the previous poster mentioned there is a utility called iSightCapture which can do this.
I once wrote a script to periodically grab an iSight image and a screencapture grab of my two monitors and post it to a private website so I could see what's going on at my computer. Here's how I scripted isightcapture to do so.
#!/bin/bash
~/scripts/isightcapture -w 320 -h 240 -t jpg ~/capture.jpg
RESULT=$?
if [ $RESULT -ne 0 ]; then
growlnotify -s -a FaceTime -m "isightcapture failed ($RESULT)" $0
exit 1
else
sips -f horizontal ~/capture.jpg
fi
screencapture -S -tjpg ~/scr1.jpg ~/scr2.jpg
RESULT=$?
if [ $RESULT -ne 0 ]; then
growlnotify -s -a FaceTime -m "screencapture failed ($RESULT)" $0
exit 2
else
sips -z 240 320 ~/scr1.jpg
sips -z 240 320 ~/scr2.jpg
fi
exit 0
Note the use of 'sips' to flip the isight capture horizontally.
A working download link (2012-10-05): iSightCapture (via), tested on OSX 10.8.2