[KevinH]

Argh here is the qimage.cpp from github qtbase master:

Simply call the save() function to save a QImage object.

    The complete list of supported file formats are available through
    the QImageReader::supportedImageFormats() and
    QImageWriter::supportedImageFormats() functions. New file formats
    can be added as plugins. By default, Qt supports the following
    formats:

    \table
    \header \li Format \li Description                      \li Qt's support
    \row    \li BMP    \li Windows Bitmap                   \li Read/write
    \row    \li GIF    \li Graphic Interchange Format (optional) \li Read
    \row    \li JPG    \li Joint Photographic Experts Group \li Read/write
    \row    \li JPEG   \li Joint Photographic Experts Group \li Read/write
    \row    \li PNG    \li Portable Network Graphics        \li Read/write
    \row    \li PBM    \li Portable Bitmap                  \li Read
    \row    \li PGM    \li Portable Graymap                 \li Read
    \row    \li PPM    \li Portable Pixmap                  \li Read/write
    \row    \li XBM    \li X11 Bitmap                       \li Read/write
    \row    \li XPM    \li X11 Pixmap                       \li Read/write
    \endtable

Note for GIF it says Read not Read/Write!

Qt really makes some silly decisions sometimes.