Claire pointed out the poor appearance of some of the 800px images used for the interactive image pages. This was caused by two factors: some where generated by the Image Markup Tool through its Web View output, which uses a default scaling algorithm that clearly isn't optimal for these images, and some images are in colour, even though they're essentially a greyscale format. I used the GIMP to re-generate all those images, converting the low-res ones to greyscale where required, but leaving the originals alone.