I have a 17" iMac that I want to hook-up a 2nd monitor to. I bought an adaptor on eBay, but the plug does not match the back of my computer. It's not a Thunderbolt or FireWire. Not sure what kind of adaptor I need or what it is called. I have a photo, but not sure how to post it here.
Unless you bought a very old iMac, its external display port is most likely mini-DVI. If you have something as old as a G4 (the "desk lamp"-looking iMacs), it may be a mini-VGA port. A helpful resource for identifying your model and seeing details of its capabilities is everymac.com - those are the only options I see for any of the 17-inch iMac models.