Welcome to the forum!
The photo is of The Angle, looking south on Hancock Avenue. If you look just to the right of the cedar tree which is just off center, you'll see the Codori barn along Emittsburg Road.
The monument on the left is the second monument of the 72nd PA regiment. The more well-known monument is the one at the wall with a soldier with his gun clubbed and upraised. There was a controversy about exactly where the 72nd was positioned, having to do with the battlefield commission that governed the placement of monuments (check this link if you want more info:
http://gettysburg.stonesentinels.co...ania/pennsylvania-infantry/72nd-pennsylvania/ ), which resulted in the two monuments).
The significance of the photo might simply be because it's the Angle. The Copse of Trees, the target for Pickett's Charge on Day 3, is somewhat visible behind the 72nd monument. Perhaps the picture was taken because it shows what's known as the inner Angle, which is the low stone wall in the foreground. I can't really see through that cedar, but I don't see the second monument to the 72nd at the wall, so this pic was definitely before 1891. Also missing is Webb Avenue, and monuments like the Armistead marker. It's a neat picture, though!
Thanks for posting, and again, welcome!