Larry at Sandwalk notes that Wellcome Library in London is making its huge collection of historical and contemporary scientific images available under a creative commons license, and passes along Skepchick's challenge to find and post "the coolest image". Others are doing it, too, so you can look around (for instance, Carl Zimmer at the Loom posts about a handsome brain and Joshua at Thoughts from Kansas shares an x-ray of a snake eating a mouse).
Here's mine: a blend of art and technology that changed the way we saw things. First, the standard representation of a running horse - a pose seen from cave drawings on up to the 19th century (this one from 1817), and then a still from the photographic sequence taken by Eadward Muybridge in 1887 (the full sequence is below):
After Muybridge's remarkable sequence of shots (taken as the horse gallopped down a track and triggered a series of cameras), no one could paint a horse - or other animal - in the same way again. Now we know what positions the legs can be in - and we can't go back and look at the old paintings the same way, let alone paint like that ever again.
Sources: Print Published: Burkitt & Hudson, London (85 Cheapside) :1 February 1817
Library reference no.: ICV No22190
Photo Published: [University of Pennsylvania, Philadelphia]: 1887
Library reference no.: ICV No49410
Full Bibliographic Record Link to Wellcome Library Catalogue
Copyrighted work available under Creative Commons by-nc 2.0 UK: England & Wales