IMAGE FORMATS
    The photo image code is structured to allow handlers for additional
    image file formats to be added easily. The photo image code maintains a
    list of these handlers. Handlers are added to the list by registering
    them with a call to Tk_CreatePhotoImageFormat. The standard Tk
    distribution comes with handlers for XBM, XPM, BMP, JPEG, PNG and
    PPM/PGM formats, which are automatically registered on initialization.