I don't get the #25 thing. Why was the photo lost?

It wasn't lost. It was in the wallet given to Walter.

Did you not pay attention to the movie? Seriously.

I think his colleague stole it. The scene were Walter discovers the neg roll is very telling.
a) Colleague acts quite suspiciously
b) Sean is obviously a hero of said colleague (he has a poster of him prominently positioned)
c) colleague has obviously read Sean's note to Walter already.

The whole film is wm dealing with the fact the he lost this image.

When Sean sent it to them, he put that negative in the wallet which he gave as a gift to Walter. Sean figured it would be a fun way to send him the most important picture, plus maybe the wallet would keep it safer. The card to the gift said "look for the gift inside", or something like that, which Walter thought meant look for the wallet within the wrapping, but Sean actually meant look inside the wallet. (Which he didn't, because the clue was kind of lame, which Sean later admits.)

So when Walter found Sean on the mountain, and he asked where it was, Sean said "you're sitting on it." Walter was obviously confused, and Sean said "it's in the wallet I gave you." And Walter's heart sank, because he had thrown out the wallet ... which he had done, ironically, because he was angry at Sean for putting him through so much grief by not sending him the picture.

But in another stroke of irony, one of Walter's pet peeves was that his Mom always kept every little thing, especially Walter's knick-knacks, refusing to throw anything away. So she saw the wallet in her trash can and kept it for him.

Well, the "looking all over for it" thing was pretty dumb. The can came in that day, and hadn't been touched until the film was trimmed, right there. The film would have been in 2 pieces, one with the exposures before #25, and a second piece AFTER 25. They should have noticed this right away, definitely before they trimmed the negs.

The whole point was that Sean had shot it(As is explained in the movie), trimmed it before sending it and then put it into the wallet he gave Walter to show him how much he meant to Sean as a friend. Walter was supposed to follow the motto of LIFE and search beyond the stuff you normally see and find the hidden things in life.

This was pretty obvious to those who watched the entire movie.

Jesus H.Christ. So what that the OP didn't understand the scene?! 

God forbid that someone didn't grasp something that was obvious to you. Congratulations on your superiority. You have all proven yourself to be the epitome of human intelligence for grasping the point of where negative 25 really was; feel free to go join your local Mensa chapter. 

Or not. In actuality, you just all demonstrated you are smug a-holes on an entertainment site. Instead of providing (what could have been helpful) insight to someone who didn't understand the plot-point, the lot of you behaved like condescending d-bags by berating him.

I agree with TreeHuggerKyle: what is it about IMDB that makes people ask like smug asses. It's a site about actors/movies/tv shows...its not about curing, say, AIDS or Ebola. So why don't y'all take your superior "I-understood-the-scene-so-why- can't-you?" attitude down by several million notches.

Seems to me that just this once we can all forgive a "stupid question." Maybe the OP just "zoned out" during the key scene in the movie. You know ... like Walter Mitty ... ?

negative 25 had been put by sean in the wallet sent to walter by sean with the rest of the roll of negatives. 1-36.