1. A system comprising: a computer system running image processing software that when executed by the computer system causes the computer system to: receive an identification of a geographical area for which an oblique-mosaic image is desired; assign surface locations to pixels included in an oblique-mosaic pixel map of the geographical area encompassing multiple source images, the oblique-mosaic pixel map being part of a mathematical model of a virtual camera looking down at an oblique angle onto the geographical area; create a ground elevation model of the ground and vertical structures within the oblique-mosaic pixel map using overlapping source images of the geographical area, wherein the source images were captured at an oblique angle and compass direction similar to the oblique angle and compass direction of the virtual camera; and reproject, with the mathematical model, source oblique image pixels of the overlapping source images for pixels included in the oblique-mosaic pixel map using the ground elevation model to thereby create the oblique-mosaic image of the geographical area.