Here are sources and details for the header images. As far as we know, all images are public domain or have an appropriate license.

• Physics - This is a segment of a bubble chamber image from the Big European Bubble Chamber at CERN. Source
• Astronomy - An image of our galaxy, provided by the Multiwavelength Milky Way project. Source
• Earth/Planetary Science - The Grand Canyon on the Colorado River, USA. Photo by chensiyuan. Source
• Chemistry - You know it, you love it, it keeps you warm, it's fire. Photo by Fir0002/Flagstaffotos. Source
• Biology - Adapted from Darwin's finch drawings, in Origin of Species.
• Medicine - A segment from Da Vinci's Vitruvian Man.
• Neuroscience - An illustration of the central nervous system from De humani corporis fabrica, by Vesalius in 1543. Source
• Psychology - An illustration of the Stroop Effect.
• Orange Tags - A segment of the 2012 Jan Satyagraha protest, nicely showing economic concerns, social gathering, and politics. Photo by Yann. Source
• Engineering - The Second Severn Crossing between England and Wales. Photo by Yummifruitbat. Source
• Computing - Metropolis. Source
• Mathematics - A page from Newtons Principia Mathematica, one of the foundational documents of calculus. Source