It is from the "my dad think's all rodeo cowboys is f***ups scene
It has to do with the scene where Jack is pretending to be riding in a rodeo.
Thanks for your replies.
In the "rodeo-fu**-ups" scene you both mentioned and in the pics JT has posted (thanks for that) he wears the lighter of his both shirts. The shirt which becomes later the shirt he finds in Jack's room together with Jack's shirt.
But in the pic I posted, he wears the other shirt; his only other shirt during their summer on Brokeback. The "darker" one, with more brown/tan instead of beige in it.
I just checked the screencaps, too (should have done that before I opened the topic and asked you, sorry). And in the scene I mentioned (Water Walking Jesus/Ain't yet had the oppurtunitiy), right before first tent scene, he wears the darker, more brown/tan shirt.
This pic is the "ain't no oppurtunity-look" (notice the shirt):

So, I assume the pic is part of this scene or an outtake/deleted shot from it. Or it's promotional/behind the scenes.
Anyway, it's a lovely picture and it's not
that important, I was just curious.
So I apologize for bothering you with my question instead of seeing through the screencaps at myself before posting.
Edit: Sorry, the pic doesn't show up. I do an attachment: