Hi Aurel,
If you download the latest Oxygen (in progress) .zip. and take a look at examples/GUI/imageWin2.o2bas. It saves a copy of the main window client area to a jpeg file at 50% quality. The system is also capable of saving to bmp, png tiff, and gif by changing the format string
It is simple only in the sense that I have produced the code after descending into the lower bowels of GDIplus hell to extract all the details
But I hope you will find it useful.
Charles