Around 2011, burned out from his corporate photography job, Theron Humphrey hatched an idea to travel the country, interview people and take their portraits in all 50 states. He quit his job, crowdfunded $16,000 and prepared to hit the road for a year in his 1994 Toyota pickup truck.

But there was one thing missing from his travel plans: a dog.

Mr. Humphrey, who was 28 at the time, had never owned a dog, but growing up in the South, hounds were always around. He typed “coonhound” into an online registry of adoptable pets, and an image of a caramel and white spotted dog with squinty eyes was the first result.

“I was like, ‘Oh my gosh. She looks like a little deer.’ I loved her speckles,” said Mr. Humphrey, who is now 42. “That same day, I drove to the animal shelter, and I went to see her, and I left with a dog. I had no idea what I was doing.”

In the years that followed, Mr. Humphrey crisscrossed the country and snapped tens of thousands of photos of Maddie, the coonhound he adopted that day. The pair traveled to 48 states, published two photography books and amassed more than 1.2 million Instagram followers along the way.