You don't. That's something for discussion among the author, editor, and book designer when things get to that stage. If you want to keep notes for yourself, that's great; but don't burden the editor with that when the editor is looking for a reason not to say "no!"
The one exception is when a particular graphic is necessary to illustrate or summarize data in an otherwise unillustrated nonfiction work. There are at least a dozen ways of indicating that; the most transparent is to add a line between paragraphs, immediately after the first paragraph that mentions that data, something like this:
\\QQ insert figure 1.1 about here XQQ\\
(that's the in-house convention for comments used at several nonfiction publishers) and include figure 1.1 as the last page of that chapter, or after the entire book if there are only one or two graphics for the whole book.
The last thing that you, as an author, want to do is appear to start imposing design choices for a "non-illustrated" book. Rightly or wrongly, many editors and book designers take this as a sign that the author is too arrogant to work with.
