What picture did Mr. Gatz show to Nick?

Mr. Gatz showed Nick a photograph that Gatsby sent him of his mansion.

"Jimmy sent me this picture." He took out his wallet with trembling fingers. "Look there."
It was a photograph of the house, cracked in the corners and dirty with many hands. He pointed out every detail to me eagerly. "Look there!" and then sought admiration from my eyes. He had shown it so often that I think it was more real to him now than the house itself.
"Jimmy sent it to me. I think it's a very pretty picture. It shows up well."