There’s a reason why they say dogs are man’s best friend. These pets are hard-working and fiercely loyal. Over the years, we’ve trained and bred dogs to help us hunt, herd sheep, pull sleds, fight crime, guide the unsighted, and protect our homes from invaders. They’re also great company. So it’s no wonder that, in the past, when someone wanted to send a message of earnest friendship and devotion, they would choose a vintage postcard featuring the image of a dog.