In the UK, new legislation (in effect from June 2014), allows the use of copyrighted images for the purpose of illustration for instruction. This means it is possible for a teacher to legally use an image without the rightholders permission when that image is essential for student understanding.
For more information see: GOV.UK: Exceptions
The image must be essential and you should only use as much of the work as is necessary to make your point.
Many creators now choose to license their work under creative commons. This asserts their rights as creator but also grants users certain freedoms.
You may be able to reuse or repurpose the work but should abide by the terms of the license this will usually include giving appropriate attribution but may also require you license your own work under particular terms.
You can search for CC licensed images in most search engines by including the name of the license in your search terms.
CC Search can help you to find reusable content from a number of popular sites:Creative Commons Search
In many instances it will be simple enough to get permission to use or reuse an image in teaching materials:
If you took a photograph or created an illustration you hold the copyright for the work and can use it as you see fit.
This does not extend to works copied from an original unless significant changes have been made, in this case the original should still be acknowledged.
You should also consider the content of your photograph, if the content was in copyright at the time the image was taken you should still seek permission from the rights holder. This also applies to images of people, where you may need to ask permission from those in the picture before using their image.
You may be given permission to reuse an image by the rights holder, this could be your own institution, a publishing house or the creator of the work. You should make sure you are aware of who the rights holder is as the creator of the work may have sold on their copyright of the image.
Be aware when seeking permission to use photographs that the rights holder is usually the creator of the work, not the subject of the photo.
Making requests in writing, stating your intent and keeping the correspondence are best practice in reducing risk.
Publishers will usually have information on reuse permissions on their website, check the terms and conditions for information.
If the copyright holder is an individual you could try contacting them directly to request permission.
Even if you have been given permission to use a work you should still include proper attribution