When writing your thesis it is acceptable to include third party content subject to fair dealing for assessment purposes. By 'fair dealing' it is meant that you should ensure that the proportion of the work used is not excessive (e.g. using the entire work when an academic point can be made from the use of an extract). This third party material could be extracts of text, quotations, illustrations, figure, photos etc. taken from books, journals, conference proceedings or web pages. All third party content should be fully acknowledged.
All theses for research degrees are, on completion, uploaded to the institutional repository. This counts as 'publishing' the thesis, so as you progress through your research journey you are strongly advised to obtain copyright permissions as you go, as this is much easier than seeking them prior to publication.
A template letter (opens as a Word document) for requesting copyright permissions is available. Keep records of any documentation relating to the granting of permissions, where no-response has been provided, and any instances where you are unable to trace or identify the rights holder. A basic spreadsheet will do the job! Also, it is good practice to record when you grant permission to others to use your work.
In some instances material will have been purposely made available for others to reuse, and a commonly seen example of this is Creative Commons Licensing which provides a standardised way to share or reuse content according to differing conditions.