The Library website has a great database full of images, Britannica ImageQuest. The photos and graphics there can be used freely and include MLA (and APA) citations, so if you use images from there, you are all set. Just pick MLA from the CITE tab below the image you chose, and copy the MLA formatted citation from the right of it:
If your image did not come from ImageQuest, you would need to create your own citation using whatever information you have, such as the creator of the image (artist, photographer, cartoonist, graphic designer), the title of the artwork or photo, the date of composition, and the location of the image. Give the actual place you saw it, either in person, or provide the name of the website and the URL.
Here are the basic examples MLA Style Center uses for citing images, seen either in person or online:
A Photograph Viewed in Person:
Cameron, Julia Margaret. Alfred, Lord Tennyson. 1866, Metropolitan Museum of Art, New York City.
A Painting Viewed Online:
Bearden, Romare. The Train. 1975. MOMA, www.moma.org/collection/works/65232?locale=en.
Even if your image came from Google Images or Pinterest, they are not the publisher. The MLA Style Center says to click on the image and then on the "Visit" button to go to the website where the image is posted, and look for a title, photographer, date, and URL there.
Here are two citations for photos found searching for "Lyme disease rashes":
Lyme Disease Rash. Photo. Lyme Resource Centre, www.lymeresourcecentre.com/info/about-lyme/lyme-rash.
“Classic” Lyme Disease Rash. Photo. CDC.gov, www.cdc.gov/lyme/images/rashes/CDC_EM.jpg.