Colorful lines on the screen. Logic board or LCD screen?

I have an iMac 17" Intel Core Duo (1.83 GHZ) that shows hundreds of red and black vertical lines on the screens. Sometimes there are white areas and the colors look somehow inverted.

I replaced the inverter but the same symptoms. There is no picture to see on an external monitor (it says "no signal".

My guess is the logic board or it might be the LCD screen. Any ideas what it could be before I invest money in a new logic board or in a LCD screen (both are very expensive and I can only buy one of the two parts)?

Also, any ideas how else I can test the logic board besides with an external monitor? And how can I test the LCD screen?

Thank you!