I just found another one that causes it: http://www.fountainpennetwork.com/fo...tralian-roses/
The third or fourth JPEG in the first post, which is 2047x700-something pixels is the problem. When I first scrolled down to it, the entire screen flickered one time, and the resized display of the JPEG in that tab was a jumble of other screen areas, but everything was otherwise ok. I changed to another tab in Firefox. When I changed back to the first tab, X froze.
I was able to SAK the X server several times, and I restarted X several times, but X would never work properly again: lightdm only displayed a mostly-black screen with square patches of white static. I even removed the nvidia module and reloaded it, but that didn't help. I had to reboot the entire system.
This is very strange, because apparently this image comes from the same forum user. What could he be doing so different with his JPEGs? I'm wondering if it's something to do with the dimensions of the image (wide and short, and 2047 is very close to the "magic number" of 2048), yet I can display much larger photos than that without a problem.
Also, these messages appeared in the ring buffer:
Nov 4 05:05:45 kernel: [138058.950126] NVRM: Xid (0000:01:00): 6, PE0001
Nov 4 05:06:43 kernel: [138116.675426] NVRM: Xid (0000:01:00): 6, PE0001