|It's worth clicking on this image and seeing the original image in Flickr as the photographer has added a really nice explanation of what it is that kidneys do and why they're rather important.|
In the recent #nhssm (use of social media in the NHS) Twitter chat, which focused on the way that the NHS might use Pinterest, a number of people queried the copyright issue (I only use CC images, with appropriate accreditation that Pinterest automates).
List of free resources
53+ Free image resources for your blog and social media posts Buffer (14 May 2014)
Public domain photos