TileMill is the desktop app for designing/styling maps. It'll export to various formats, but primarily geared for upload to Mapbox's hosted map service (free and paid plans that are well worth it imo - just a happy customer).
For std image files, Mapbox make source available for their TileStream server (https://github.com/mapbox/tilestream). Not sure if/when it'll get support for the raster images.
