Image citation rights permit the use of a photo in scientific presentations and art reviews. Always ask the author for permission, however, if you are copying and using a photo, a musical composition or another similar artwork in your thesis or slideshow. You can link freely.
There are photos and music in the internet that you can freely use, presuming that you mention the source. You can, for example, use Flickr, Pixabay, FreeFoto.com, Freeimages.co.uk, Freeimages.com, Pics4Learning and NASA's resources. Take a look at University of Eastern Finland's list for more (link descriptions are in Finnish).