Version control for graphical objects is rare in the open source world, but it exists in the commercial world. Autodesk Drawing Compare, which is part of Autodesk Vault, is an example. For film and game work, there's Alienbrain. A game has many non-text assets - maps, motion capture data, 3D models - and they all get revised. The systems which can pull all that together can display, and difference, many graphical formats.