Definition of citation in English: noun
A quotation from or reference to a book, paper, or author, especially in a scholarly work.
Definition of quotation in English: noun
A group of words taken from a text or speech and repeated by someone other than the original author or speaker
Words (above) are referenced directly from Oxford Dictionaries online here and here respectively.
Citing and creative commons
When using creative commons (cc) images from Flickr a simple attribution like the one on the right should be evident on the last slides of credits.
If used online the hyperlink can be linked directly to the title or creator.
This site explains how to do create a proper citation.
THIS site does it for you. (Though not so nice for slides used to make presentations with the text directly on the image.)