This work implements the Poisson equation formulation of the Retinex model for image enhancements using a graph hardware accelerator performing finite difference updates on a 2D lattice graph PE array. A single clock gating control signal manages the data flow, data sharing, and reuse pattern among neighboring PEs during massively parallel updates. With increasing user-configurable update count, image noise and shadow can be progressively removed with the inevitable loss of image details. Accommodating a non-overlap image mapping scheme in which a $20\times 20$ image tile can be processed without external memory access at a time, the proposed accelerator consists of 18 $\times 18$ regular PEs surrounded by $4\times 20$ boundary PEs with reconfigurable data flow and 4 boundary cache registers. Fabricated using a 65nm technology, the test chip occupies 0.2955mm² core area, and consumes 2.191mW operating at 1V, 25.6MHz, and a reconfigurable 10- or 14-bit precision.