The ranch clearly bears Steve's surname, and we can assume since Diana is alone in the picture that she was visiting after his death.
Though it remains unclear exactly how the ranch was started and why there are no horses pictured behind Diana, writer-director Patty Jenkins did provide some insight into the photo. After a fan on Twitter asked for more details about the photograph, Jenkins replied: “When Diana moved to the US, and all her friends were now long gone, she went and visited Steve's families ranch, where he'd grown up. All she has left are those memories.”
From Jenkins' response we now know Trevor Ranch was both owned by Steve's family and was seemingly still in operation after his death. The films and comics leave Steve's upbringing ambiguous, so this is an exciting new detail to add to the Wonder Woman mythology. It is not specified whether or not Diana is immortal, but Jenkins also revealed in an interview with Collider that Diana is at least 800 to potentially over 1,000 years old. The true tragedy of Diana's life is that those she grows close with both romantically and in friendship, she outlives.
Though, if Diana is indeed immortal, it leaves a nice amount of room for Wonder Woman sequels in the coming years.
