Most of the library databases and the IUCN Red List will have suggested citations or will generate citations for sources, but many websites will not. Referring to the MLA Handbook is always to best bet, but ZoteroBib does a pretty decent job in getting you started.
The 9th edition of the MLA Handbook Appendix 2 has several examples of works-cited-list entries and the examples of citing fine art and still images can be found on pages 331 - 333. The online MLA Style Center also has examples of image citations.
Here is an example of how the following photograph (found through Pixabay) should be cited using the MLA style.
stokpic. Blonde Girl Taking Photo. 10 February 2015. Pixabay. pixabay.com/photos/blonde-girl-taking-photo-629726/
This work (Blonde Girl Taking, by stokpic), identified by Pixabay, is free of known copyright restrictions.
Remember, the library has a copy of the Handbook at the Reference Desk if you need to use it.