The characteristics of an imaging system formed by a left-handed material (LHM) slab of finite length are studied, and the influence of the finite length of the slab on the image quality is a nalyzed. Unusual phenomena such as surface bright spots and negative energy stream at the image side are observed and explained as the cavity effects of surface plasmons excited by the vanescent components of the incident field. For a thin LHM slab, the cavity effects are found rather nsitive to the length of the slab; the bright spots on the bottom surface of the slab may stretch to the image plane and degrade the image quality.