When referencing images that come from articles, it is perfectly acceptable to cite the parent article. If you want to specifically cite the image, use the article citation and make the following changes:
Creating Image Captions
From the College Art Association - A full caption includes, whenever available and appropriate, information in the following order:
Captions provide titles or explanatory notes. When creating captions, ask yourself the following questions.
Guides for writing captions for Figures and Tables: