I have photos I am using in a slide presentation at an educational conference. Do I need to seek permission.

This is a question that invokes the "fair use" exception to copyright infringement. In a non-profit setting where multiple copies are not being made and there is no obvious economic detriment to the copyright owner, such a use may be okay. It is impossible to state a categorical rule with respect to "fair use" because all such determinations are intensely fact-dependent.