GaussSum saves images as .ppm files. This stands for Portable Pixel Map. These files are similar to .bmp or Windows Bitmap files in that they are uncompressed. As a result, they are quite big, typical 0.9MB per image for the images produced by GaussSum. (This file format was used because it appeared to be the only common demoninator between Linux, Windows, Tkinter and Gnuplot - .png support is on my Things to Do list.)

In Windows, such files are best viewed using IrfanView. This superb free utility may also to used to convert .ppm files to the smaller .png ("the better version of .gif"). For example, I can convert an image of a geometry optimisation from .ppm (901KB) to .png (4.19KB) or .gif (4.65KB).

In Linux, you can use ImageMagick to convert the files, e.g. "convert GeoOpt.ppm GeoOpt.png".