Digital aerial photos (like this TerraServer image of Necedah, WI) are called Digital Orthophotos (DOP) or Digital Orthographic Quarter Quads (DOQQ). The images are corrected for optical distortions or irregularities in the Earth's surface to correspond with true positions on the ground. The free images from TerraServer may be up to ten years old and are usually taken in a leaf-off season. They are grayscale infrared photos, causing conifers and water to appear as dark gray or black. More recent photos and color images are available from commercial vendors. You can also scan paper aerial photos (but ortho-rectification requires specialized software).