What are HTC system images for? HTC recently released a Nexus-One system image for Gingerbread. What do I do with that? Can I use it to update my phone to "stock" Gingerbread (no carrier modifications?) If so, how? Or is it just some kind of backup?
It's more or less a backup version of the Gingerbread release for the Nexus One. HTC manufactured the Nexus One, so they host a copy of the kernel, drivers, system images and so forth that are released for it, just like they do for the other phones they made.

You can use it to flash back to a stock ROM if you own a Nexus One, but if you have a different HTC phone it won't do you much good. You might be able to flash it, but it would most likely either not run properly or brick your phone. I certainly wouldn't recommend it.