Onboard cameras are an immense sensing asset for spacecraft, especially for the application of spacecraft navigation. Stars, small bodies and extended planetary bodies make up the imaging targets readily available to spacecraft in space. This dissertation makes use of the ellipsoidal extended bodies (i.e., planets and moons) and their conic section projections and features in images for spacecraft navigation. Modeled as conic sections, we make use of Jupiter's atmospheric bands for latitude estimation, exploit a planet's terminator for sun direction estimation, and apply Saturn's rings to position estimation. Furthermore, ellipsoidal bodies also serve as potential imaging targets for camera calibration. This dissertation details the first camera calibration algorithm that uses a single imaged ellipsoid and enables spacecraft camera calibration using a nearby planet or moon. Considering navigation and calibration as sub-fields of state estimation, we provide four contributions where planetary images serve as tools for spacecraft state estimation.