Cite the still the way you would any image from a website. The following provides an example:

Still of the queen from the film Mirror Mirror. “Excuse This Rant about Mirror Mirror,” by Emmet Asher-Perrin, 2 Apr. 2012. Tor.com, www.tor.com/2012/04/02/excuse-this-rant-about-mirror-mirror.

In the example above, the Title of Source element is a description of the still you are discussing. The first Container element is the essay in which the still appears, followed by the author and the date of publication. The second Container element is the name of the website, and the Location element is the URL.

If you are referring to an essay on a website that contains many stills from a film, you can cite the essay as a whole instead of an individual still:

Asher-Perrin, Emmet. “Excuse This Rant about Mirror Mirror.” Tor.com, 2 Apr. 2012, www.tor.com/2012/04/02/excuse-this-rant-about-mirror-mirror.

See our related post on citing images on websites.