I discovered Reddit’s “Without a Trace” forum, which has links to loads of MP articles I’ve missed, and have been going through them this night. In the process I found this article about Donna Michele Barnhill. A quote from it:
The photo released in 1981 of Donna Barnhill was that of a school picture when she was between the age of 7-8 years old, the news release stated. In the mid 2000s, the CUE Center for Missing Persons located additional grainy snapshots, including an “eye-shut” photo. Barnhill’s family did not purchase the photo from the school because her eyes were closed, and the photo was of poor quality.
In other words, the picture the NCMEC has on their poster was taken nearly half a lifetime before Donna disappeared at age 13. The only other photo is of such poor quality that there’s no real way to tell what she looked like when she vanished.