My cat died two years ago, now my dog won’t stop staring at his portrait

This cat was my best friend my entire childhood, when he disappeared at an old age about three years ago now, I knew he had gone somewhere to die. I was distraught, so my friend painted me a portrait of him. I’ve had it hung up in the same spot since.

I got my dog, Stu, a year ago. He never met my old cat, and this portrait has been hung up in the exact same place the entire time I’ve had him. Tonight he was barking softly at the portrait, staring at it, when he’s never paid it any mind before.