Copyrights and permissions question...

Can someone help me learn more about copyrights and permissions for using images in our courses in iTunes U? For example, is it permissable to use an image from Wikimedia Commons freely? How should a tag be placed on an image (can you collect all the references to images and put them on one page, or do they need to appear with the image?).

If you are including a Prezi, "licensed for educational use only," can you include images from the internet through their Google image search engine (selecting "images licenced for commercial use")?

Thanks for any clarification that can be provided.