Quantization operation in the imaging process rounds the measured values of intensities into integers. It neglects the possible differences between the intensities having the same rounded numbers, and therefore lowers the grayscale resolution of images. Although some methods have been proposed for the reconstruction of high-grayscale resolution images from multiple subpixel-shifted low-spatial-resolution and low-grayscale-resolution images, the problems of nonsmooth transition within regions and insufficient intensity levels still exist. A grayscale superresolution method based on fill light and a photographing apparatus are proposed to deal with the problems. The photographing apparatus can add fill lights with slightly different intensities to the captured images without changing the brightness of scenes. Our reconstruction method is based on the method of estimating a float number from several rounded integers. Then, a high-grayscale-resolution image is reconstructed from multiple low-grayscale-resolution images with slightly different intensity fill lights. Simulated data and real-world data have been used for the evaluation of the method, and the experimental results show that our method effectively improves the grayscale resolution. Besides, our method is convenient for a graphics processing unit implementation.