The following are all covered by copyright:
To comply with copyright law, even within educational exceptions, you must always include a sufficient acknowledgment when using still and moving images. Think of this like referencing a text - if you did not include the reference you would be plagiarising another person's work.
A sufficient acknowledgement should include the following:
For example: Julia Margaret Cameron, The Mountain Nymph Sweet Liberty, 1866, The Metropolitan Museum of Art
Image databases, like Bridgeman and Artstor, often include an acknowledgement for you to use which can be directly copied and pasted into your work.
When using an image from Flickr or Creative Commons, the name of the person who has uploaded the image can also be used. This is usually included in the details section of the image.
In order to comply with copyright the integrity of the image or film must be protected. 'Integrity' refers to the representation of the work as the maker or artist would have intended. This means that no significant editing or changes should be made to the appearance of the original image. This also means that if using a detail from a work, this should be stated in your acknowledgement.