The dog serves as both its blanket and mattress. One thing which can be deciphered from this picture is that the dog is extremely well behaved and well mannered. It does not put up any sort of resistance against the advances of the cat, if anything, the dog is actually allowing it to climb up all over its body.
This is what we call true friendship. They both empathize with each other and realize that if they re to move forward as friends, then they will have to unite with each other. Become friends and live happily ever after, or end up enemies and constantly watch over each other's shoulders. And it's a neat arrangement because the two of them provide heat to each other, this is especially useful during cold winter days.