A security fix in Leopard and 10.4.10 prevents the app iSightCapture from working unless it is run by the console user. Here is a workaround to get things going again. Create a simple AppleScript with the following command as the entire file:
do shell script "/your/path/isightcapture imagename"
Replace /your/path/isightcapture with the full path to the iSightCapture program, and replace imagename with the name you'd like to use for captured images. Save the script as an application, and call that program when you want to capture the iSight's output.
This forum thread over on the macosx.com forums contains more details, including info on how to set up a cron task to automate screen grabs, as well as a way to stop your little program's icon from showing up in the Dock.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080126190204997