Question: A lot of websites and presentation books suggest that I buy stock photography for using in my slides, do I have to use other peoples images or can I use my own?
Answer: While there are thousands of images on the internet, sometimes the perfect image is a photograph that you took. The big downside of some stock photography is that is looks too posed or fake or that everyone uses the sames images and thus they lose their power and impact.
As a general rule, as long as the image is of sufficient quality and is relevant to your message then use it.
I had an example of a person who wanted to talk about hieroglyphics who looked for images on the internet but found none that suited his talk, so instead he scanned in his own hand-drawn images and used them. They were relevant and they had the added advantage of being unique and thus probably more memorable.