Wherever you are obtaining third party content from, it is important that you provide proper acknowledgment of the source within your teaching/training materials, just as you would in any scholarly materials that you produce. Free image websites such as Pexels, Pixabay and Unsplash often say that although attribution is not necessary, it is appreciated.
Materials made available under open licences such as Creative Commons or the Open Government Licence will have very specific requirements when it comes to correct attribution and you can find out more about that in the sections of this guide specifically devoted to those licences.
When relying on a statutory exception, such as Section 30 (Criticism, review, quotation) of the CDPA, one of the requirements is that your use of the work must be "accompanied by a sufficient acknowledgement".
When negotiating permission directly with copyright owners or their agents they will often specify a particular attribution statement as part of the licence agreement.
