Yes, it's abundantly clear that this image in the poster is fabricated, staged, etc. There's no question that this image exists outside the normal context of the film. This is one reason why it's so interesting to me.
I just watched BBM tonight... and while I was watching I paid special attention to Ennis's jackets. He seems to wear the same jean jacket throughout the years (much in the same way that he seems to wear the brown corduroy jacket for many years). The jean jacket that he wears (seen in the drive-in movie scene, the fireworks scene, the "three-hands" scene, etc.) has a brown/ tan lining on the inside of the collar. The tan collar... is exactly the color I associate with Ennis. In the poster, it's interesting to me that the collar is turned up. My continuing question is about whether the jean jacket in the poster is ever seen in the film. And, because the collar is turned up... it's almost impossible to tell.