Most sources require you to include an acknowledgement directly under the image or other media. e.g.

Leonardo da Vinci. Ginevra de' Benci [obverse], Painting.
Ailsa Mellon Bruce Fund, 1967.6.1.a. Courtesy National Gallery of Art, Washington.
If you need to cite the image, provide:
Artist’s Name. (Years of creation). Title of the artwork [Type of artwork]. Where you retrieved the image from.
Type of artwork could be: Chart, Diagram, Drawing, Graph, Illustration, Image, Map, Photograph, Painting, Sculpture, etc.
Examples:
CC Search Search for Creative Commons images, music, sound effects or video via CC search tools: Europeana, Flickr, Fotopedia, Google, Jamendo, Open Clip Art Library, SpinXpress, Wikimedia Commons, YouTube, Pixabay, ccMixter or SoundCloud.
Let's CC One-click search for Creative Commons images, sounds, video and docs via Flickr, Jamendo, ccMixter, Youtube and Slideshare.
Warning Don't assume that results from CC Search or Let's CC can be legally used for your purposes. Verify that the item you want to use has a suitable CC license by following the link provided to the site on which it was originally used.
YouTube Videos can only be downloaded if they have a Creative Commons license. To find CC videos, click the “Filter” tab at top left and look for the “Creative commons” option, or use CC Search. To view licence details for a video, click the "Share" button, then look for "SHOW MORE". Some also allow remixing and will link to a video editor.
Khan Academy Videos can be used under a CC BY-NC-SA licence. Provide the attribution statement “All Khan Academy content is available for free at www.khanacademy.org”.
Academic Earth Free educational videos from the world's top universities.
TED-Ed Educational videos created by TedEd collaborators or by teachers who have added supplementary content to YouTube videos.
Videezy Free background video footage, mostly scenic.
The following databases have good collections of images that can be used for educational purposes:
The Creative Commons search tools CC Search and Let's CC do not cover all image resources. See below for specialised, publicly available sources with more refined search options or try the suggested Ara library databases listed in the box at right.
Jump to: Art Galleries & Museums, Photographs, Clip Art, Other Public Domain Image Sources.
Art Galleries & Museums
Most images from the galleries below can be used for educational purposes but check the "terms of use" for each gallery and also check for restrictions on individual images.
Other Public Domain Image Sources