Whether you are blogging, creating a presentation, writing for publication or designing a poster, you are likely to want to illustrate your material with other people's images on occasion.
Bear in mind that free-to-view images are not necessarily free to re-use. Even uncredited photos on transient websites may be protected by copyright. If the creator or host has not stated that they allow further use of the image, you should ask for permission to reproduce it. If you don't get a reply, you shouldn't go ahead.
If you intend to reproduce an image in order to critique or review it, you may not need permission: UK copyright law permits you to 'quote' from copyright material, including images, providing your use is 'fair dealing'. Essentially, this means your copy must have no impact on the market for the original image (e.g. a lower resolution, or a cropped version), and you must credit the rights-holder.
In 2014, European courts ruled that linking to third party material does not necessarily infringe copyright. To incorporate a digital image into your own website, consider whether you can embed a link rather than copying it.
Third-party copyright images which are integral to your work as a teacher or student may be legally defensible as "Illustration for Instruction".
Typical educational scenarios in which you may not need the rights-holder's permission to utilize their image:
Your image caption or credits should reiterate any copyright statement or licence terms indicated at the source.
Ensure that your material is not shared outside the classroom (physical or virtual), or with anyone other than the markers in the case of assessed work. Or look for images which are licensed for re-use in an educational context (see opposite).
Be aware that even your own photos of artworks and panoramas may not be risk-free: although an artist's copyright may have expired (usually 70 years after their death), the gallery may have a 'no photographs' policy which doesn't exempt educational use. Or the country where you took the photo may impose legal restrictions on the reproduction of copyright material sited in public (including France, Italy and Greece).
Corporate logos can be particularly problematic, as they are likely to be trademarked as well as protected by copyright. Ensure that any logo which you reproduce for the purpose of 'instruction' doesn't leave the (physical or virtual) classroom. Or contact the company for permission to use their logo in your educational material.
The creator of an image may release it with a Creative Commons licence, which provides a simple, standardized way to grant copyright permissions to their work.
Many image libraries allow you to filter search results by licence status: for instance CC-0 (in the public domain, no attribution required), CC-BY (free to re-use with attribution) or CC-BY-NC (free to re-use in a non-commercial context). The CC-Search tool enables you to search multiple libraries simultaneously.
University of York liaison librarian Ned Potter has blogged about the best sources of CC-0 images for academic work, including presentations, posters and websites (2018).
Antonella da Messina 'Portrait of a Man'. About 1475-6. National Gallery NG1141.
A reproduction of work by a long-deceased artist may still be protected by copyright - the creators of the digital image may claim the right to license its use. Check out the University Library's guide to History of Art resources for York students and staff:
See also the University Image Library (managed by Marketing): images of campus and student life for use in University of York materials.
Biochemistry student in the lab © University of York
A source to treat with caution:
Be aware that Getty actively pursue unlicensed copying of their images and will invoice the website owner. US legal practitioner Steve Schlackman has shared some Tips for Responding to a Getty Images Extortion Letter (Art Law Journal, 2014).
Wikimedia Commons brings together a very comprehensive list of answers to the question "Do copyright laws allow the upload of pictures of...?", in the UK and other countries.
The UK's Intellectual Property Office has published a Copyright Notice (2014, pdf) for a general audience, providing advice about reproducing digital images and photographs, and protecting your own images.
UK-based art dealer Follio.com has published a straightforward infographic (2016) illustrating How to Avoid Copyright Infringement when manipulating images. Note the differences between US and UK law which are outlined in the guide.