image source: Wellcome Images

Great online resource - http://images.wellcome.ac.uk/

there is also a library that you can visit:

Wellcome Library
Part of Wellcome Collection
183 Euston Road, London NW1 2BE, UK
T +44 (0)20 7611 8722
F +44 (0)20 7611 8369

Wellcome Images
T +44 (0)20 7611 8348
F +44 (0)20 7611 8577
E images@wellcome.ac.uk

They regularly have interesting exhibitions and events here, worth visiting.

Check rules for using images from any such a collection. Remember to reference all images.