LIFE images dating as far back as the 1750s are now available from Google! To limit your search to the LIFE photo archive, just add "source:life" to your Google Image search box along with your keyword(s).
Suggested search terms for neat science-related images, both historic and contemporary:
science
DNA
chemistry
nuclear
earth
microscope
math
space
astronaut
medicine
brain
electricity
microwave
time exposure
human body
and on and on and on...