The Brillouin equation for information processing, stating the entropic cost of erasing bits. The paper is "Galactic Gradients, Postbiological Evolution and the Apparent Failure of SETI" by Circovic and Bradbury, arxiv.org/abs/astro-ph/0506110
I took the photo when I was not paying attention during a lecture. Not paying attention = no thermodynamic costs. Taking this photo could also in principle have been non-entropic; the real cost occured when I erased the picture on the camera chip after transferring it to the computer.