we hooked up a second monitor and that one showed perfectly. He seems to think it is the display itself, repair cost $780!
(OT, but this is why I'm not a fan of "all-in-one" computers and am a Mac Mini owner.)
You might reconsider my suggestion to buy an external display, if you have room for it. You could even "upgrade" to a larger external display and get more screen real estate for a few hundred dollars.
There may be a third-party repair service available at lower cost, though I haven't found it yet. Such services
do exist for Mac notebook computers and charge about half of what Apple does. For example here are two for the MacBook display:
"Overnight Screen Replacement Service for MacBook Series"
http://techrestore.com/xcart/product.php?productid=17384&cat=273&page=1
And here is another service:
http://www.dttservice.com/macbook.html#macbook2lcd
Perhaps by contacting them, they could advise you about iMac repairs.
I note that TechRestore does do G5 iMac LCD panel replacement, but their listing for Intel iMac repairs is blank. The G5 link is here:
http://www.dttservice.com/imac/imacg5.html#imacg5%20lcd