If you want an inexpensive cover for a wall, purchase flat white sheets and dye them to match your furniture, tack them to the wall with a staple gun and some decorative ribbon. If you pull them tight, they look wonderful. When you get tired of the color, or decide you can do something different, then they are easy to remove. Pictures on walls should add to the ambiance of the room. If you want a cluttered, homey look then hang many pictures, if you are striving for a more modern look then one or two large prints is ideal.
Most rooms have four walls.
As long as they all have a similar frame and colors then they will go together well. The key to a professional look is making sure that all the elements in a room go together and lead the eye to a focal point, no matter where you are in the room. It could be a collection of pictures on a side table, a fireplace, or an attractive lamp. Don't be afraid to experiment with furniture arrangements and various moveable items. Use a focal point to draw the eye away from the flaws in the room.