Links
This page contains links to resources that you may find useful. Standard disclaimer: We are not responsible for the content of external links.
Sites with public domain images, that you might find useful when creating card sorts packs, etc
Flickr: The site’s search options allow you to specify the type of licence you want.
Wikimedia Commons: Has a structured index of images, plus site search.
Sites with interesting visualisations
Sketchplanations has single-image cartoons illustrating a wide range of concepts, including common types of error.