The x-ray imaging performance is reported using polycrystalline lead iodide as a thick semiconductor detector on an active matrix flat panel array. We have developed a test image sensor with 100 micron pixel size in a 512 X 512 format, using amorphous silicon TFTs for matrix addressing. The new 14 bit electronic system allows radiographic and fluoroscopic x-ray imaging. PbI2 has larger x-ray absorption and higher charge generation efficiency than selenium, and has the potential for higher sensitivity imaging. The films are deposited by vacuum sublimation and have been grown thicker than 100 micrometer. Measurements of the carrier transport and charge collection, together with modeling studies show how the x-ray sensitivity depends on the material properties. Imaging measurements find excellent spatial resolution and confirm models of the x-ray sensitivity. Both radiographic and fluoroscopic imaging are demonstrated. While good overall imaging is obtained, the dark leakage current and image lag need further improvement.