Images as perceived by human eyes or recorded by cameras are usually optical patterns with spatially varying intensity or color profiles. Besides intensity and color, the information of an image can also be encoded in a spatially varying distribution of phase or polarization state. Interestingly, such images may not be viewed directly by human eyes or cameras as they may exhibit highly uniform intensity profiles. Here we propose and experimentally demonstrate an approach to hide a high-resolution grayscale image in a square laser beam with a size less than half a millimeter. An image with a pixel size of 300 nm by 300 nm is encoded into the spatially variant polarization states of the laser beam, which can be revealed after passing through a linear polarizer. This unique technology in hidden grayscale image and polarization manipulation renders new opportunities for a diverse range of applications including encryption, imaging, optical communications, quantum science, and fundamental physics.