Many images on sites such as Flickr are uploaded under a Creative Commons licence that allows users to use, reproduce and/or adapt the material. It is always important to check the terms of the licence before using these images: there are different levels of usage permitted, and some licences require that you make any resources you have created using these images freely available under Creative Commons.
You can use the Creative Commons Search facility to search sites such as Flickr, Google Images, YouTube and Fotopedia for Creative Commons-licensed images and videos.
There are also a wide variety of websites on the internet that provide copyright-cleared or public domain images for us - see the box to the right for a selection of these.
You will always need to obtain permission of the copyright owner in order to take a photograph of a copyrighted work, unless the work is a sculpture or other 3D piece of artwork on public display. A photograph is considered a derivative work, and copyright law solely invests the right to create derivate works in the copyright owner.
You may take photographs of works that are in the public domain (for artistic works, the duration of copyright is the lifetime of the creator plus 70 years), regardless of who owns them. However, be aware that many galleries and museums have terms of entry that prohibit or restrict photography of exhibits.
All the webpages linked below contain images that are in the public domain or have been licensed for free use.