How do you choose a citation style for images? Consider the following:

• If you have been instructed to use a specific style for your assignment, use that.
• If this is a casual presentation such as a short class slideshow or personal project, attribution is probably enough.
• Do you want to include information about art objects (such as medium, dimensions, or museum collection info) in your citations? Chicago/Art Bulletin may work for you. Chicago Style is standard for art history.
• APA Style was created by the American Psychological Association. It is widely used in behavioral and social sciences, as well as many scientific disciplines. It may not be a good choice for your art history paper, but is great for HPSS papers.
• MLA Style is maintained by the Modern Language Association and is standard for fields like literary studies and media studies. It is a flexible format, and used a lot at RISD.

Why so many styles? You might as well ask why there are so many different types of birds in the world. It's evolution. People working in specific fields had different needs, so they created new styles. You can always ask the nice people at A&L or the Research & Instruction Librarians for help if you are feeling overwhelmed.