OWL - The Online Writing Lab (OWL) from Purdue has instructions for formatting citations. Most CCS undergraduate classes require MLA style citations (although OWL has information on APA, Chicago, and more). OWL provides guidance on citing all types of resources, including books, websites, and artworks, among others.
zbib - This site generates basic citations for you - just enter the URL of your source. You will need to check the result against the MLA Handbook or OWL, however, as these citations are not always formatted correctly.
Citing sources for visual works is just as important as it is in a written context. You must not only record the creators of works you have consulted for inspiration, but you should also document the evolution of your own ideas. This allows you to build stronger work by analyzing your earlier iterations and also demonstrates the organic evolution of your work if there are ever concerns about the academic integrity of your work.
"Common knowledge" is information that does not need to be cited because it would be known by the average reader (i.e. Barack Obama was the first African American president of the US). If you are not sure if your information qualifies as common knowledge, verify with your professor who your audience is. For more information, see the MIT Handbook.
You must cite your sources - both within the text of your paper and in a "works cited" page - whenever you include information that is not common knowledge. Many undergraduate classes will use the MLA style (see the OWL link above). For in-text citations, you typically only need the author's last name and page number (if it comes from a book or an article) inside parenthesis at the end of the sentence. Your works cited page (or bibliography) is where you provide more in-depth information about each of your sources.