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.
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.
UK copyright law permits you to 'quote' other people's images, provided that they are relevant to your discussion or critique (i.e. not just used for decorative purposes), and that you have used no more than is required for your specific purpose.
You also need to abide by 'fair dealing': your use of the image must have no impact on the market for the original (you should use a lower resolution or cropped version) and you must fully acknowledge the rights-holder (your image caption or credits should reiterate any copyright statement or licence terms indicated at the source).
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:
The captions for any images you reproduce should provide information about the image and its source in accordance with your department's preferred referencing style. You should also identify the rights-holder (e.g. " Copyright © University of York"), reiterate any copyright statements or licensing terms indicated at the source (e.g. "All rights reserved", or "This work is licensed under a Creative Commons Attribution 4.0 International License") and state whether you have obtained any necessary permissions to use the image.
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 CC0 (in the public domain, no attribution required), CC BY (free to re-use with attribution and indication if changes were made) or CC BY-NC (free to re-use in a non-commercial context, with attribution and indication if changes were made). 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 CC0 images for academic work, including presentations, posters and websites (2018).
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:
The following image of Monet's 'The Water Lily Pond' is licensed for reuse by the National Gallery under the terms of a Creative Commons CC BY-NC-ND Licence (fully credited and used for noncommercial purposes only, without further modification). The image caption provides full attribution in the Chicago referencing style (as used by the History of Art Department: see Referencing Styles - a Practical Guide) and also identifies the rights-holder and licensing terms used at source:
^ Claude Monet, The Water-Lily Pond, 1899, oil on canvas, 88.3 x 93.1 cm, National Gallery, London. https://www.nationalgallery.org.uk/paintings/claude-monet-the-water-lily-pond. Copyright © 2016–2021 The National Gallery. This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
A source to treat with caution:
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.