Q. Can I use third-party images to illustrate my thesis?
A thesis is treated as an examination for copyright purposes, which means that you can use third-party materials provided that the work is correctly acknowledged and that the amounts used come under 'fair dealing' limits.
However, these permissions don't extend to publication of third-party images in a repository and permission will therefore be required.
Under these circumstances, the law permits limited (fair dealing) use of a copyright work for the 'purpose of review and criticism', provided that it is accompanied by sufficient acknowledgement and provided that the work has already been made available to the public.
The copying must be genuinely connected with review and criticism of the work or another work and not purely for illustrative or enhancement purposes. There has been very little case law testing the criticism and review defence. Whether the defence applies to theses is very much down to interpretation.
If you own the copyright in the images you are free to use them.