I was under the impression that Mark has implemented code to automatically break up the images behind-the-scenes, so you might want to post this in the bug reports forum.
I'll wager 4096 is the magic number for your particular card, before it starts screwing up... Powers of 2.
I tried 4200x600 pixels and I get a lot of trash on screen.
I'll wager 4096 is the magic number for your particular card, before it starts screwing up... Powers of 2.