After the press conference I asked Mike A'Hearn a couple of questions about the raw images we're seeing online. First I asked about the way that most of the images seem to be divided into 4 quadrants. Mike told me that each quadrant of the detector is processed through a different amplifier, and each amplifier is a little bit different. When they calibrate the images, this effect is corrected for, and the 4 quadrants will all look like they have the same exposure settings.
Also, I asked him whether all of the images that we were seeing were actually targeted on the comet. He said yes, and then he said "so you're probably going to ask me why the comet is not showing up in some of them." He reminded me that the MRI has many filters, some of them very narrow-band, which means that they only permit a very narrow range of wavelengths of light through to the detector. In specific, he said, they are often using an ultraviolet filter that is specifically designed to look for the OH- radical (that is, the hydroxyl ion, which forms when water molecules are split). He said that when there are outbursts you can see the comet through this filter, but that when the comet is quiet you might not see anything at all. Unfortunately, the website doesn't say which images are taken through which filters.
So, I am guessing that these images, from the Medium Resolution Imager, were all taken through different filters, but I don't know which ones.