Geez people, I think I've figured out the solution!!!
Same problem as above, everything just went haywire and I was getting all of these Cannot Modify Header messages. It was always in the nggallery.php file. I'd deactivate plugins, tons of error messages, just a big fat mess. Then I'd think I would get it fixed, get rid of the error messages and then the problem of no left image, per exposia's screen shot. I'm on Mac and Firefox 3.6.3, btw.
What fixed it (reading lots of threads here and kind of putting everything together):
1. Go to the nggallery.php file in the nextgen plugin folder.
2. Open that with whatever you use to edit files (I use page spinner) that doesn't add a bunch of crap. I guess Notepad in Windows.
3. Go to the bottom of the file and the end is:
4. Use your arrow keys to see if there are blank spaces beyond that, if so, use backspace and get rid of them.
Now this part is weird, but I ran into it here, regarding a messed up config file (and it's in the WordPress help files)
5. Put your cursor between the question mark and whatever that other thing is called. I'll use a dollar sign to mark where cursor should be:
6. Press your DELETE button, not the backspace, but find DELETE. It's going to delete that pointy thing. Hold it FIFTEEN seconds. Then type the pointy thing:
Sorry, don't know what that's called.
Then save your file (I save directly back to my FTP, but do it however you do it)
Apparently somehow a space is being put in between those two characters and is mucks it all up. I did this and like magic, I can now edit thumbs again, and all those horrible error messages are gone.
This sounds a LOT more complicated than it is. Once you get to the file and do the delete thing, it's quick and painless.
Hope this helps somebody...I've been tearing my hair out all afternoon.