I have a never-before-published picture of Freddie Mercury (approx. 15 ft. away, and taken by me) from Queen's Dallas concert on March 22, 1975. Would it be profitable to sell glossies (whatever size) of the pic online, and would marketing it prior to the movie Bohemian Rhapsody debuting in early November be a good idea? Do glossies even sell these days? How should it be priced? Back in 2008 I asked the Rolling Stone magazine if they would be interested in the picture, but I never received a response.
1975 is a long time ago (though I remember the year too), but this SE question is worth a read. If I'm reading Wikipedia's "Sheer Heart Attack" page correctly, it sounds like the McFarlin Memorial Auditorium would be an indoor private venue, so it could come down to whether you could reasonably expect to have permission for the photograph and copyright of the work.
Whether it would be profitable might end up being a Catch 22. You make nothing on the picture, who cares? Long time ago, right? But the general rule is that the moment anything makes a significant profit lawyers will start crawling out of the woodwork. And there goes any profit.
[A passing thought - this might be why Rolling Stone didn't respond. Or it may be that they already have publishing rights to more pictures of Freddie than they can use.]
Short answer - careful with this. A previously unseen shot of Freddie might be big news. If there's a chance lawyers might get involved, get one first.
If the image has anything unique about it compared to what is already out there you are probably better off contacting music magazines like rolling stone (again, because the people have changed and it might be more topical now) or agencies like getty and sell them some/all rights. Another option would be to contact galleries specializing in rocknroll memorabilia.
Note that with both of the above you have basically zero bargaining power unless you can solicit offers from more than one and have them compete for exclusivity (again, assuming the image actually has something original or exceptional to it)
Another choice would be to contact Queen fan clubs, online communities etc. and offer limited edition prints through them to their members.
General purpose sites like etsy, redbubble or society6 are probably the least favorable option unless you have a marketing campaign bringing targeted demographics to your listings or have managed to create an iconic design out of your image to be printed on cellphone cases bags shirts etc.