TL;TR; Pictures got corrupted while transferring them to Linux box.
@gerhard I was 100% sure my PNG files are valid and correct, as I was able to open them on my Windows computer. However your hint pushed me into right direction and I verified the PNG files how they looks like on the Linux box. And you where absolutely right, as all PNG files got damaged. Root cause was the way how I transferred them. I used SCP (WinSCP) and PNG files got transferred as text files, leading to the corruption. On my new attempt I transferred everything as 7z archive and just extract them directly on the Linux box. This time the import was working fine.
Much appreciate your support, pointing me into right direction 
While my import is working now for 99,9% of all post, I have one more question. Is there a way to transfer the status for solved topics as well? On the new prod forum I plan to use discourse-solved. Question how to transfer the old status into new world?
Again: Much appreciate your support.