There shouldn't be any problem, as long as the source is disclosed in a footnote or an endnote (eg, see the photos on this website.)

Both the Mahon Report and the Chippendale Report are on this website, and they both contain some good photos.

Archives NZ holds all the evidence produced to the Royal Commission and will provide copies on a PDF file. I'm not sure how they deal with photos, but they may have electronic copies that can be emailed. They charge a small fee, which can be paid over the internet by credit card.