Looks like something is missing?

Its looks like there is something missing above the optical drive in an iMac I have. I'm new to imacs and when I bought this one, the guy said apple told him it needed a new power supply. So I bought and replaced that, no power. I took it back apart and I found no hard drive. I also bought two g5s from the same guy so I just pulled the hard drive out of one and put it in the iMac Intel. Still won't turn on. It looks like there is something missing in the top right corner of the iMac just above the optical drive. Can anyone tell me what, if anything, goes there?