Well, for those of us with inquiring minds that need to know... the answer is pretty much as expected. I set iScreenShotIndex=2147483647 (0x7FFFFFFF), started Skyrim on the left monitor, and started pressing F12 while watching the screenshot folder on the right monitor. The screenshots are taken in this order:
In other words, Skyrim is incrementing a signed integer by one so it rolls over into negative numbers as indicated by the minus sign. After it finally reaches -1, it rolls over to 0.
So, yeah, I seriously doubt anyone would take 4,294,967,295 screenshots in BMP format.