I was just reminded by Nick Hopwood on an email-list that the Wellcome Trust has quite recently introduced a more generous policy for the free use of Wellcome Images. The reproduction price is now waived for a wide range of non-commercial uses. See the terms here.
The large majority of the >100,000 (!) online items in the digital image database are classical medical history pictures. But the number of contemporary images is growing rapidly. Now you can choose among 40 molecular models, for example this molecular model of a prokaryotic ribosome (credit: MRC Lab of Molecular Biology, UK, and Wellcome Collection):
This colour-enhanced scanning electron micrograph of a 2-3 day old human embryo (credit: Yorgos Nikas, Wellcome Images):
can now also be freely used in academic publications, theses, university museum exhibitions etc. There are over 21,000 contemporary images altogether. I guess you need permission if you want to manipulate them though.