An explanation of captions can be found starting at 3.21 (p.136+) in the CMOS 17E.
Captions should be capitalized using sentence case.
Captions appear outside, and usually below, the illustration.
Titles for paintings, drawings, photographs, statues, and books will be italicized and in title case.
Titles of articles, essays, and short poems are not in italics, but will be in quotation marks.
Commit to either full sentences with appropriate punctuation in the caption or incomplete sentences with no closing punctuation.
Illustration numbers (e.g., Figure 3, Plate 1) should be followed by a period, a space, then the caption.
If the text of your caption runs long, it should be single spaced.
Figure 3. Life of Chinese Emperors, Qing period