BIODIC provides access to a wide variety of electron micrographs in the field of Biology. Categories include Anatomy, Animal Biology, Cell Biology, Plant Biology, Monera-prokaryots, Fungi, and Protists.
According to JSTOR, "Global Plants is the world’s largest database of digitized plant specimens and a locus for international scientific research and collaboration." You can search by location, collection, plant type, etc.
Images of the universe, our planet, and astronauts created by NASA and available through the Internet Archive. Images are generally copyright free (for the few exceptions, read the "terms" section of the website).