Tips for Windows users:

If your serial interface is COM1 the program w2000a-screenshot.exe can be startet without
any arguments. Pushing the Quick Print button on the oszi twice will start the screenshot 
immediatly. If using another port it has to be given as argument to the program:

w2000a-screenshot.exe -c 4 

This will instruct the program to search on COM4 for the oszi. If You are using a Prolific 
USB to RS232 converter, problems with the driver might occur. In this case the port
can't be opened by the program. I solved the problem by installing an older driver version.
Download and instuctions (in german) can be found here:

https://itler.net/prolific-usb-to-serial-comm-port-error-code-10/

 
Converting the image format:

The screenshot program creates images in PPM format. This is a simple non compression format.
The image can be converted by a graphic program (like IrfanView). I'm using the command line
tool mogrify (by ImageMagick) which I copied into the screenshot directory. The command

mogrify.exe -format jpg *.ppm

converts all PPM images in the directory into JPG format (without overwriting the originals).
How can I get the program? Go to the Homepage of ImageMagick, download the ZIP archive and
uncompress it.

ftp://ftp.imagemagick.org/pub/ImageMagick/binaries/ImageMagick-7.0.10-10-portable-Q16-x86.zip

or 

https://imagemagick.org/download/binaries/ImageMagick-7.0.10-10-portable-Q16-x86.zip 

Then copy the program mogrify into the screenshot directory