What kind of bad pic?
The G4 lamps are old enough now to be developing problems inside their LCD monitors. You can find threads about troubleshooting the monitors and there are some caps or controller boards that can be replaced. But first, try an external monitor (via mini-VGA port) to see whether the GFX chip is the problem or not.
PSU can also be a problem if it has not been kept clean - it can gather fluff and overheat. If you don't want to get inside the machine, you can first try vacuum cleaning with a rubber tube around the grille at the bottom of the neck. Best is a thorough disassembly and clean of all parts. You can find good visual guides online with a simple search.
Beware that common cause of GFX chip failure is that the baseplate of the lamp has not been kept properly tightened. The baseplate is the heatsink for that chip and several others. If the chip loses firm contact with the heat-transfer pad on the base, you can lose the chip. Then you will have to search for a new motherboard unless you are a professional - replacing the chips is not for amateurs in my (amateur) experience.
Rick