Before we get all caught up in the discovery of the photograph of Joseph Smith, let's consider the rationale of the person who didn't let on to the LDS historians who questioned her that it even existed: Joseph's wife, Emma.
It tracks well that she would not have handed this picture over to those who followed Brigham Young to the Uintah Basin. Young defamed and condemned her from the pulpit in General Conference because she always refuted the LDS claim that Joseph practiced polygamy, and because she refused to hand over Joseph's manuscript of the JST Bible, and also because she refused to come to Utah in support of the Brighamite sect.
Being a source of deep embarrassment to Young, Emma's disapproval of his successful bid to take leadership of the larger portion of the LDS movement bugged him for the duration of his presidency. His response to this refutation and disapproval was to nurture a very bitter grudge.
So of course Emma kept it hidden. She likely also instructed its inheritors to keep it thus. Why allow an authentic image of Joseph to be cross-branded ad nauseum with all of the lies foisted upon his reputation? Why trust any sect after a schism with anything that is real?
For anyone who finds that they are still in agreement with Young on the matter of Joseph beginning the practice of polygamy (and that Emma, therefore, should be considered a liar), Denver Snuffer skillfully dissected volume 9 of the Joseph Smith Papers in September of 2020 for you as if it were a damned frog:
http://denversnufferpodcast.com/transcripts/124_The-Foolish-and-the-Wise_Transcript.pdf
For myself, I do not consider Emma a liar (or a capitulator, or a poisoner) at all. I think anyone who insists as much should read Snuffer's examination of the Institution's own presentation of the "Nancy Rigdon letter" in their continued miscarriage of their own history.
The Church botching the record of events has become almost ritualistic at this point, which makes perfect sense when you remember that Jesus insisted that we "cannot serve two masters". In choosing to honor Brigham Young and their post-Joseph hierarchy, the institution indeed has thrown Joseph, Emma, Hyrum, and the entire Smith family under the bus.
It is no surprise that Emma would not follow them, serve them, or share with them. Our admiration of her should only increase if the daguerreotype is deemed to be real.
