Manuscript Preparation, Copyediting, and Proofreading
Q. This problem came up when copyediting a journal: on a page that is occupied by a broadside image and has a single footnote (to the caption of the image), should the footnote be oriented the same as the image and caption (i.e., 90 degrees from normal), or should it stay as regular?
A. It’s not usual to footnote a caption rather than simply run the note into the caption with “Note:” in front of it, so you won’t find a rule for this in CMOS. But to compound the oddity by having two strings of text at right angles to each other is probably not a good idea. It’s almost always best, if possible, to set all the type on a page in such a way that it can be read continuously without turning the book.