As professionals and artists, we would be concerned when someone – professional or amateur – uses our material. Certainly, plagiarism is not nice when it happens to us, and as professionals, we work to avoid it and credit every source of information that we use.
This applies to ALL professionals. If we use audio, video, or images in our slide shows, we should respect the artist who worked to create the material and only use their work with permission. This means acquiring the appropriate license or using the correct accreditation.
This article is a perspective from a photographer that is timely and pertinent. http://petapixel.com/2012/01/10/this-photograph-is-not-free
You can see more of John B. Mueller’s work here.