Do I always need permission to use an image in my thesis or dissertation?

When using any material that is protected by copyright, including photographs, diagrams, text quotations etc., the default is to get the permission of the copyright owner.

However, depending on what you are including and for what purpose, you may have different options.

Many materials, including images, may already been licensed for reuse. For example, many images are under a Creative Commons licence which allow you to include them in your thesis under the terms of the licence.
Do not assume that because an image is free to download from a website it is also free to reuse. Unless there is a licence or copyright statement allowing you to use it, you must assume that it is protected by copyright and that permission is normally necessary to include it in your work.
In some cases, it may be possible to use an image without permission. Copyright exceptions, defined in UK copyright law, allow use of materials, including texts and images, for specific purposes including quotation, criticism and review and education. Using these exceptions might include, for example, directly critiquing an image, 'quoting' the image in the context of your discussion or including it as part of an exam answer. Please seek advice from the copyright team if you are considering relying on an exception.
In many cases and particularly for open access PhD theses, you will be advised to clear permission. Visit the UCL copyright website for more information on clearing permissions.