Hi Joenade, sorry you're still having trouble connecting your Cintiq 13HD to your 2012 iMac and my solution(s) have only been more confusing.
First of all, please ignore my earlier post about what sequence the Cintiq and it's adapter are plugged in. In my case, this turned out to not have any bearing on what otherwise was a successful solution.
Second, my solution is unique to the 2008-2009 MacBook Pro's with a single pre-"Thunderbolt" mini diplay port. I believe your 2012 iMac has two later "Thunderbolt" ports with which to connect peripheral displays(?). This Apple port and plug design, now two generations old, are confusing, as they are visually identical and I don't believe can be swapped for Cintiq use now, if ever. My 4 year old MacBook Pro requires an additional USB connection for it's HDMI adapter, -Your Thunderbolt to HDMI Adaptor Should Not. Make sure, however, that the adaptor you are using is in fact the Thunderbolt design and supports at least 1080p resolution. All mini display adaptors, HMDI or otherwise, are notoriously bad, or simply do not work. I highly recommend the kanex iAdapt V2 for your use. (Please click on that kanex link to see that HDMI adaptor I beleive apropriate for your use.)
I did have a belkin mini display (which was mistakenly the later Thunderbolt design) to HDMI adaptor fail causing my Cintiq screen to appear streaked or damaged. After calling Wacom, they called attention to the likely adapter simply failing and recomended the kanex, among other brands that are known and tested by Wacom to work with the Cintiqs.
Our Cintiq 13HD screen tablets had worked presumably well connected to our respective Macs BEFORE we updating their operating systems to OS X 10.9 Mavericks. Among the many other "improvements" in this upgrade ironically, is improved "multiple display support". (Additionaly, but not critial to this issue at hand is, the also inhanced "Mission Control" feature. It now allows for virtually "switching" between Desktop workspaces independently between multiple displays, BUT is not, as yet (as I understand it), compatible for use with screen tablets and MUST BE TURNED OFF in Mission Control Preferences in order to use any of the Cintiqs as we have before.)
Mission Control's switchable features and preferences aside, I believe it is Mavericks enhanced multi display "support" that is now too smart for our connected displays good. In other words, Mavericks is now looking for a much more specific kind of connection for displays in order to better support them. I also believe this support is more oriented toward passive displays, and is axaserbated with "stylus" screen tablet connection with much more complex interaction between the user, screen display and computer. This new circumstance I believe yields a unique "resolution conflict" between our Cintiq and the new OS X 10.9, crashing your iMac, or in my older MacBook Pro's case, blinked off and on, intermittently for irregular amounts of time.
Aside from making sure your HDMI adpator is Thunderbolt and 1080p compatable, it may likely being necessary to reset your NVRAM and PRAM (at start up, before the appearance of the gray screen, hold down the Command/Option/P/R keys simultaneously until computer restarts again for second time) My apologies for stating the obvious if you already know this reset, however intel-based Macs store NVRAM memory and PowerPC PRAM, including critical "Screen Resolution" data. After any display resolution conflict, it's always good to reset these now likely corrupted memory settings.
I hope this better describes the "connection" (HDMI adaptor) issue I believe is causing display "resolution conflicts" now more likely in the upgraded Mavericks and it's "improved multi-display" working environment.