can i insert an image to be the background of my 2d game if so how ?

There might be cleverer ways to do this, but I'd start with a big box in the background with a material on that renders your texture!

If its a side scrolling game, make the box bigger than your camera (so you don't see the whole thing at once), and move it gradually sideways as the player progresses from one side to the other. This'll give the impression that the background is moving more slowly than the foreground, and results in a nice paralax effect that gives a proper feeling of depth to your scene.