Barbara: Take a look at this link http://creativecommons.org/about/licenses/. It provides a legal interpretation of the different licence conditions.
If you used a photo in a PowerPoint presentation, the first question is where did it come from. All OER materials have either a Creative Commons licence or a unique copyright restrictions based on the institution you downloaded it from. The safest way to intrepret copyright is to assume (which is legally correct) that everything that is published, be it text, video, audio, photos or graphics automatically is protected by copyright. The copyright owner must grant you permission to use those materials and may restrict how they are used. Thus the need for a CC licence since it eliminates the need to always go to the copyright owner before publishing, reusing or re-mixing.