Apparently it's not possible (or not discoverable after several minutes of banging around the Annotation tool) how to crop a screenshot. This is so basic I'm sure I must be missing something. The "Select" toolbar icon selects only annotations, but it took me a while to figure that. Initially I thought it selected a rectangular area of the image, to crop to it. As a suggestion, the tooltip for that icon could read "Select annotation" (instead of "Select").
*** This bug has been marked as a duplicate of bug 430465 ***