What's the reason of this display trouble?
Hello. Please see the link.
17" IMAC's display had some trouble.
What's the reason? LCD or graphic card?
What parts need for repair?
This looks like a faulty graphics chip on the logic board. You have 3 options to repair this:
What I would do in your situation is attempt the reflow first to see if it works. If it doesn't then I would just replace the logic board. See this guide: iMac Intel 17" Logic Board Replacement
If you want more info on reflowing or reballing try a google search of GPU reflow diy, BGA rework.
I hope this helps, good luck!