Most of the images at BIL are available free of charge for non-commercial purposes (like teaching), but buying commercial-use rights is also possible.
I don’t really agree that the library is aimed at students (teachers, lecturers – maybe), since personal subscriptions are $293 per annum and I could imagine that institute subscriptions are not cheaper. UPDATE: See comments below. To me a corporate client is a main target of this initiative, but I may agree that relatively high subscription rates may be dictated by the nature of the images – most of them come from experiments (almost no computer generated illustrations).
Microstock photography sites hold quite a lot of science-related images, however their scientific value is very often zero (as an example, search for DNA). It is overall nice idea to have a scientific equivalent of a microstock, but I would rather leave the subscriptions out and release low quality (for web/computer screen use) material under non-commercial Creative Commons license.