There are plenty of ways to change this:
If you want one big background (stretched) then you open an image editor such as paint.net and stretch the image and save it manually. Alternately find a background with a higher resolution.
If you want a background that you can adjust the resolution of and have it stretch whenever, you have to use a sprite (or other similar plugin, such as a 9 patch). However note that this is less computationally efficient, and drawing to the screen is what takes up the most processor time.
If you want a background that fills the screen and never moves, then set the background's parallax to 0 and make sure the source image and tiled background object are both the same resolution as the screen.