Pictures at the Royal Society. Some good ones for botany & biology at least.
They expect payment for most uses but do allow some free use but you must apply to them.
"Data visualization is a running theme of visual literacy here, and Manuel Lima has been one of its biggest advocates since 2005 when, shortly after graduating from the Parson School of Design, he launched VisualComplexity - an ambitious portal for the visualization of complex networks across a multitude of disciplines, from biology to history to the social web."