What are the differences between the Geoserver stores ImageMosaic, WorldImage, and ImagePyramid? I noticed that ImagePyramid is no longer included in the latest Geoserver. Is one of these more efficient than the others?
WorldImage is for accessing regular images that have a world file associated with them. ImageMosaic is for serving up multiple raster files (usually organized into tiles) as a single layer. And ImagePyramid is essentially a multi level mosaic in which higher levels are generalized versions of lower levels.
Usually people don't serve up world images directly... as they are not an efficient format for geospatial, because for instance they are usually not tiled internally so to read a small section of the image usually means reading the entire image file. GeoTiffs (which can be tiled internally) are much better suited. For people that want to serve large amounts of raster data usually a mosaic or pyramid of geotiffs is the way to go with GeoServer.
You will find some information in the docs explaining the distinction:
ImagePyramid does not come out of the box. You have to install an extension:
ImageMosaic = Stores more than one image - ImageMosaic data store allows the creation of a mosaic
WorldImage = Georeferencing file (plain text file with coordinates and pixel resolution)
ImagePyramid = image pyramid builds multiple mosaics of images, each one at a different zoom level, making it so that each tile is stored in a separate file. Can speed up image handling.
All documented here: http://docs.geoserver.org/latest/en/user/index.html