When the author or creator of a work, in this case an image, is not listed, the citation should include whatever you are putting first in your works cited page. In this case it would be "Advertising notice announcing the sale of slaves." This is done so the reader can easily located the bibliographic information for this image in your works cited page (the same reason the author's last name is used in other situations)
You can use the image's full name the first time you cite it in your text and then use a shortened version for subsequent mentions so it doesn't get too repetitive and long.
So, the format would look like this.
The sale of slaves began in America ("Advertising notice announcing the sale of slaves").
And then for the following ones, like this:
After the Civil War, slavery was outlawed in America ("Advertising notice...").
Obviously since this is an image, a page number is not needed.
Hope this helps! Good luck!