They perhaps don't 'need' to mention this photo as being taken hours before her disappearance, but I'd be interested to see another missing person case where police had an hours-earlier photo in their possession and did NOT identify it as such. Anyone?
How would we know this? In my county, LE puts out pictures of missing persons all the time, with no caption whatsoever. Some of them look as if they could be quite recent - but they never give the time of the picture. Some look like selfies or yearbook or prom or DMV photos (so I'd exclude that). But some of the photos show people doing things they might have done that day.
I will say that I don't think SBCSO does the kind of job that, say, Park Rangers do at Grand Canyon in updating and clarifying what they have released. A lot of people have been vocal with the Park Superintendent over the years, to get GCNP to be more forthcoming (they were always more forthcoming, at least since I began researching and working there circa 1985).
Here's an example where it's clear that the photo is designed to indicate what the missing person was wearing, and with a little inference, it's clear it's not "day of."
Missing Person Search at Grand Canyon - Grand Canyon National Park (U.S. National Park Service)
By contrast, the information that SBSCO has released is scant, and it doesn't give clues that indicate that the photo was taken any other time. Indeed, it appears to have been cropped out of a larger picture and is aiming to show what kind of clothing and hat Barbara may have been wearing.
Also, I'm going with what the VI has said about photos, and I trust his repeated statements on this topic, which seems to heat up about every other thread. We will have to each use our own intuitions on this, I think. One would think that LE could adopt either policy (always say if it's a recent picture - but of course, then they'd need forensic study before saying that) or just do their best to get pictures out there.
It is possible they'd tell a reporter, if the reporter was from a reputable news agency.