In “The Thing Itself,” from The Photographer’s Eye by John Szarkowski, I think the difference between the thing/subject that is shown in the photograph than the photograph itself would be that, in a photograph you are only seeing what the camera is seeing and not the whole story. For example if the photographer takes a picture of a subject the moment they smile, the subject would then be seen as having a good life and enjoying their self, but that wouldn’t be the case in reality because we only know what we were shown and the subject after that because they could actually be angry, sad, depressed, struggling, ect. But we wouldn’t really know.
I think that a photograph might be more important than the thing itself because photos/pictures can speak a thousand words, where as being in the moment with the thing might not be that interesting. Pictures last for along time and can express many things. As the days go on, so do we, and we just keep on going. On the other hand pictures will always be there to tell a story.
I would describe what you are saying in the first paragraph this way: a photograph excludes what is outside of the frame and by this exclusion can really change the meaning of what is in the frame. A famous example of this was a photo of Obama bending his head down. He looked dejected when the photo was tightly cropped. When the photo was not cropped, you could see he was bending his head down to listen to someone shorter than he is. What was outside of the frame in the fist cropping totally changes the meaning of the photo.