Cite an image from a slide presentation on the Web the same way you would cite an image on a Web page (MLA 6.79). Indicate the slide and its number, either at the end of the entry, or in a parenthetical citation in your text:
Creater's last name, First name Middle initial (if any). Title of the slide presentation. Year of creation. The name of the website, publisher, URL (with http:// or https:// if you submit your paper online). Slide number.
Benton, Thomas Hart. Instruments of Power from America Today. 1930–31. The Met, Metropolitan Museum of Art, www.metmuseum.org/blogs/now-at-the-met/from-the-director/2012/benton/slideshow. Slide
See also "How do I cite a slide presentation uploaded by an instructor to a learning management system" at MLA.org
If the presenter is the author created the image in the slide:
Presenter's last name, First name Middle initial (if any). "Title of the image". The name of the presentation or the name of the class, the date, the location. Indicate the slide and its number in the optional-element slot at the end of the entry, or in a parenthetical citation in the body of the text:
Smith, Ryan. “Goals of the Course.” English 204: African American Literature, 4 Apr. 2016, Evergreen State College, Olympia. Slide 2.
If the presenter did not create the image in the slide, provide information about the original, if known:
Monet, Claude. Bridge over a Pond of Water Lilies. 1899. Introduction to the History of Art, taught by Jane Ford, 4 Apr. 2016, Bates College, Lewiston. Slide 2.