The copyright exception for research allows you to use images in an essay, as it is considered an unpublished work*. This only applies within the assessed learning space; you don’t have the right to then share that work in a public space, i.e. repository, website, blog, YouTube, etc.
You can use CC-licenced materials as long as you follow the licence conditions.
*If you are intending to publish your work, you will need to explore the possibilities of this with your module/programme leader or supervisor, and will likely need to request permission from the owner for each image you wish to use.
The amount of images, or non-textual content, you can copy is "restricted to fair dealing, which rules out unfair or unreasonable uses such as copying a whole film for “research” instead of buying the DVD. Use of a copy should be accompanied by sufficient acknowledgement (e.g. in a reference or bibliography)" (IPO, 2014).
Understanding Copyright by 
Dear images art, copyright and culture by 
Copyright law for artists, photographers and designers by Creative Commons (CCL) is a licensing system set up to help image creators distribute their work.
An image has a licence attached indicating how it can or cannot be used.
There are four elements to the licensing system and this information can be found alongside the photo on the website if a CC licence has been applied.
The four elements are applied in different variations to form six licences.
The latter is an overall public domain licence (CC-0) which means the image has no copyright attached and is free to use on all accounts.
