If you do a search in the Search tab page of IDL Online Help for the string "Image Pyramids", you will quickly get to the "Image Tiling" article, which briefly discusses this. I do not know whether this is precisely what you are looking for, but, generally speaking, you could use an array of IDL Structures to create a hierarchical tree of related images, alternatively an IDL pointer array. In either case the images pointed to by your pointer array or by a field(s) in your array of IDL Structures can have whatever size you want. The programming trick remaining is to determine what "protocol" you will create that will encapsulate how the different images are interrelated. James Jones