You may be able to access images or photographs freely on the internet or in printed information, but that does not mean that you can copy or reuse without permission. They are all protected by copyright as "artistic works".
You would need to check who owns the copyright of the images and be aware that some companies, like Getty images are extremely proactive in protecting their rights to the extent of actually searching for infringement and demanding royalties.
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 (e.g reference it correctly).
Creative Commons, or CC, Licenses are a simple way of letting people know what the rights holder is happy for them to do with the work. Think of them as an extension to what you can already do under copyright law: for example, any restrictions imposed by a CC License, or the complete absence of a licence on a work, cannot override your right to use reasonable amounts of a work in an academic context.
There are several different licences, with names like CC BY, CC BY-ND, and so on, sometimes with a version number attached, e.g., CC BY-ND 4.0 If you find a work (a photo, video, webpage, or whatever) that references one of these licenses by a statement such as "This work is openly licensed via CC BY 4.0" then you will probably be able to re-use the work, but you will need to consult the details about Creative Commons Licenses to find out exactly what you can do, as there are some restrictions:
BY: If the licence includes BY, it means you must credit the original creator of the work when you use it.
ND: This means you cannot adapt the work at all - no cropping photos for example (Note: you might be allowed to edit an image under a copyright exception).
NC: Non-commercial - you cannot make money out of re-using the work.
SA: Any adaptations you make of the original work must be given the same CC License as the original.