We systematically investigate lensing of electromagnetic waves by a negative refractive-index material slab constructed from a two-dimensional photonic crystal with properly designed equifrequency-surface configuration [Luo et al., Phys. Rev. B 65, 201104 (2002)]. We find that a point source placed in the vicinity of the slab can form a good-quality image in the opposite side of the slab. However, the image is strongly confined in the near-field region of the slab and gradually degrades and disappears when moved beyond the near-field domain. In addition, the image-slab distance has little dependence on the source-slab distance and the slab thickness. On the other hand, the image can also form by a slab with a positive effective refractive index. We have analyzed the equifrequency-surface contour configuration of this photonic crystal and found that the overall imaging properties of this photonic crystal slab are dominantly governed by the self-collimation effect and complex near-field wave scattering effect, rather than by the all-angle negative-refraction effect.