A nonlinear revision control system for images is designed with the common digital editing and sketching workflows in mind. DAG (directed acyclic graph) is used as the core structure and DAG nodes represent editing operations, and DAG edges indicate the corresponding spatial, temporal and semantic relationships. DAG is visualized in RevG (revision graph), which provides not only a meaningful display of the revision history, but also an intuitive interface for common revision control operations such as review, replay, diff, addition, branching, merging, and conflict resolving. Beyond revision control, the system also facilitates artistic creation processes in common image editing and digital painting workflows. A prototype system has been built upon GIMP, an open source image editor, and demonstrates its effectiveness through a formative user study and comparisons with alternative revision control systems.