As example, here's an advertisement that was found on this web page.
Luckily, the title and creator of the ad are provided in its caption: the ad is called "Wish or Work" by Hal Depuey. So here's how this advertisement's citation would probably look, following the rules/template of MLA style. We'd name the creator of the ad first, then the title of the ad (in quotation marks, since it's a small work), then follow that with info. about the essay/web page in which the ad was found.