Be careful about what you do with images. They are considered other people's intellectual property. This means that you should cite any images you use, just like you do with books and articles, and that some images might have restrictions on how you can use them.
Some cites, like ARTstor, have a citation generator that will format the citation for you. If you use the citation generator make sure to double check that the information is correct.
As a student you can use images from ARTstor and the VRC for the work that you turn in for school, or for your own inspiration and information. You cannot post these images publicly online or distribute them. If you find an image that you want to use for a publication or to share more widely outside of class, you must get the permission of the copyright holder. For more information see the Licensed Resources Policy.
flickr is a great tool because it allows people who upload images to make a copyright designation. Under Creative Commons Licensing the copyright holder determines how an image can be used by other people. To learn more about Creative Commons: http://creativecommons.org/licenses/. You can search Creative Commons licenses, by type, by using the "advanced search" feature in flickr.