Here is the only clue to the final July 4th picture you can find in the novel. It's mentioned on page 191 (Chapter 33).
“In that instant, kneeling there, everything came clear to him… In those few seconds [Jack] understood everything. There was a certain black-and-white picture he remembered seeing as a child, in catechism class… a jumble of whites and blacks… Then one of the children in the third row had gasped, "It's Jesus!" …. "I see Him! I see Him!" … Everyone had seen the face of Jesus in the jumble of blacks and whites except Jacky… when everyone else had tumbled their way up from the church basement and out onto the street he had lingered behind… He hated it… It was a big fake… [But] as he turned to go he had seen the face of Jesus from the corner of his eye… He turned back, his heart in his throat. Everything had suddenly clicked into place and he had stared at the picture with fearful wonder, unable to believe he had missed it… Looking at Jack Torrance. What had only been a meaningless sprawl had suddenly been transformed into a stark black-and-white etching of the face of Christ Our Lord. Fearful wonder became terror. He had cussed in front of a picture of Jesus. He would be damned. He would be in hell with the sinners. The face of Christ had been in the picture all along. All along.”
He's describing this very popular black & white photo that floated around in the seventies where you have to find the hidden image of Jesus. Either you see it or you don't.