background picture in my game

Import the image as a texture, and then create a material that uses that texture. Then create a 'plane' primitive object, remove the collider component (assuming you don't need it), and assign the material to the plane.

Technically, the built-in plane may be more highly tessellated than is actually needed for this, but it's most likely the easiest and quickest solution. (An alternative would be to make your own 'quad' mesh in a modeling program.)