Don't know if there is a resolution limit.
Here's something to try. Import a few pics to your computer. Then use a USB flash drive & the camera connection kit.
Plug the USB flash drive into your computer & create a new folder titled DCIM. Then put your movie/photo files into the folder. The files must have a filename with exactly 8 characters long (no spaces) plus the file extension (i.e., my-movie.mov; DSCN0164.jpg).
Now plug the flash drive into the iPad using the camera connection kit. Open the Photos app, the movie/photo files should appear & you can import. (You can not export using the camera connection kit.)
Secrets of the iPad Camera Connection Kit
I had the same problem, also with D600 jpgs after upgrading to IOS6. iOS5 worked fine.
Then I found out, that the problem dissapeared after i had upgraded iPhoto on ipad.
I use iPhoto 1.1 now. I also upgraded other apps, but I assume that iPhoto was the responsible one.
So if somebody could try and report please.
Before I had exactly the same problem, jpgs bigger as about 5000 pixels were not transferd from the camera connection kit, it ended with a red question mark.
The problem is also discussed here: https://discussions.apple.com/thread/4347990?start=15&tstart=0
Thank you very much for your support. I do have IPhoto 1.1 and large files do not seem a problem per se, as the import of RAW (.NEF) works flawlessly. Nevertheless your e-mails somehow led me to following:
I deleted all "used" or "open" apps and then restarted my IPad.
After that I was able to import some jpg's made with full resolution and fine compression set (26.2 MPx, around 13 MB). So it might be an intermittent problem and I'll keep trying. If it happens again I will first try to "close" the Photo app completely - who knows?
after what I have read about this problem so far, it sounds like a good idea, to "close" open apps when the problem reoccures.
So far it works fine here. By the way, the other app I updated was "photogene"
Importing raw and jpg was never a problem on my iPad either, but then I started putting raw and jpgs on different cards which the d600 allows and then I found out, that I could not import the fullsize jpgs!