Some embodiments provide a non-transitory machine-readable medium that stores a program. The program reads a file representing a source image. The file specifies an interior image and a set of successive exterior images that correspond to a set of successive zoom levels. The interior image includes a plurality of pixels. Each pixel in the interior image has a particular size. Each exterior image in the set of successive exterior images includes a plurality of pixels configured to encompass the interior image. The plurality of pixels of each successive interior image have a successively larger size than the particular size. The program generates the source image based on the interior image and the set of successive exterior images. The program receives a selection of a zoom level in the set of successive zoom levels. The program generates a target image based on the selected zoom level and the source image.