Update:
I just found this page: http://www.advancedcustomfields.com/add-ons/options-page/
Sounds like this feature needs to be purchased separately now (it used to be included in the version that I had first installed on this site).
I figured out how to fix my problem by adding the field to a page, rather than it’s own separate entity (even though the field is associated with the header, not a page).
Seems to work fine, or at least I hope!
Thanks again!
Sharon
