Using wooden panelling, concrete or plaster finishes for walls will add much more warmth than a flat finish such as a matt paint or as alternatives to traditional wallpaper. The way light plays across the organic changes in texture, or the rise and fall of the surface, immediately makes the room feel richer.
If you want a simpler seasonal change, adding lots of framed photos on a mantel or window ledge in varying shapes will eradicate cold empty space without looking messy. When looking at how to arrange pictures on a wall to add warmth, consider the proportions of your room and height of the ceiling; but keep the scale large and theme them, such as black and white photographs, or painted unframed oils so they don’t look chaotic. More is more here; a weedy single image in a frame lost in space will add to the feeling of airiness or coldness, not reverse it.