1. A method of providing virtual microscope slide images to a user computer, the method comprising: storing a data structure of digital images of a specimen on a microscope slide, the data structure comprising a first series of contiguous image tiles at a first magnification of at least a portion of the specimen, a second series of contiguous image tiles at a second, higher magnification from the portion of the specimen, and coordinate information defining the spatial relationship between each of the image tiles in the first and second series; providing only a subset of the available image tiles from the first series to the user computer; and in response to receipt of a magnification selection input, using the magnification selection input and the coordinate information to retrieve an image tile for a magnified portion of the specimen from the second series; and providing the retrieved image tile from the second series to the user computer.