I was going to update the previous entry, but what the heck: Emily Lakdawalla at the Planetary Society blog just posted more images of the MRO with more explanations too. Funny, one crater she decided to highlight is one I saw as well, but had already grabbed the one I displayed in the last entry, and didn't feel like doing another. I figured what the heck, let people just go scroll through the images (I'm lazier than Emily; she posted several images on her site). But man, you need a good connection. The images really are incredibly big.
Hmph. The first astronomical camera I used had 384x512 pixels (about 1/5 of a megapixel), if I remember correctly, and that was one of the biggest ones you could get. The detector used by MRO's camera HIRISE is actually 14 separate chips, 2048x128 across, and then it takes a series of images on a swath as the satellite orbits the planet (like running a paintbrush down a wall; the narrow brush makes a long path). That's how it makes such enormous images, and the nice optics of the telescope gives it such phenomenal resolution. Pretty cool stuff.