If you may see a ‘green screen’ when you launch Photo Booth or iChat (or any application that uses the built-in iSight camera) on your iMac G5 (iSight), be sure to update to Mac OS X 10.4.3. You can use Software Update to download and install the update, or download it from Apple Downloads. Be sure that you shut your computer down completely (not just restart) after installing the update. iChat and Photo Booth should behave as expected after this.
If you can’t update right away, try quitting and starting the application again. If the issue continues after updating, disconnect any external video cameras (including webcams, TV tuners, DV cameras, or DV converters) that may be connected via FireWire or USB and try again.