[math-fun] Changing photos into matrices

Does anyone know a way to turn black and white jpeg images into rectangular arrays of pixel-by-pixel grayscale levels? My Mac has Adobe Photoshop CS6 and Mathematica, but I can't figure out a way to get either of them to do it. The reason I want to do this has to do with this year's Gathering for Gardner, so I believe it (just barely) qualifies as a math-fun topic. Jim Propp