CSIRO Science Image provides a curated set of over 4000 photographs from the CSIRO archives, covering the full breadth of Australian science. These images are free to download and use under a Creative Commons License (as they have been on this site!).

The images come with a lot of metadata, and you can search by topic, by spatial location or by photo characteristics (color, size, orientation etc).

CSIRO has a regular Image of the Day post on Twitter and Facebook - just search for #CSIROPics.