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.