There is a routine called COMPST that interleaves two images and scales
them to each use half the color table. It does no alignment and requires
that you have or develop appropriate color tables. The calling sequence is
IDL> imageout= compst(image1, image2)
It is currently set up to work with 512x512 images. It is a good idea to massage the images so that they emphasize the intensity ranges of interest, e.g., for low-intensity features, a decompressed image gives poor results. Creative color tables may also be useful.