I have the same Problem. I think the solution in the link only works for Nprinting 16.X. It did not work mer neither. I'm kind of frustrated at the moment. Considering the fact that Nprinting is a REPORTING TOOL to generate REPORTS non the less there is no option to set the image quality is some sort of really, really bad joke.
Think about it for a moment.
When more then 5 years ago i had a similar issue i have adressed it by making sure that my images are not getting scaled/zoomed/streched.
As i was using QlikView app and Pixel Perfect template i went to chart properties in QlikView --> Caption and there i have set the size of the chart to be as big as the required output (say 800/600). Then in my Pixel Perfect report i used the same size for image nesting.
Very often we are trying to zoom charts which are small and its quality gets worse. In other templates you may not have full control of the chart size but its quality should improve.
so maybe make this chart bigger in your source (to start with) and test again.
Regarding the other solution above for clipboard zooming you need to do this on NPrinting Engine QlikVIew Desktop using NPrinting user as it is a user setting for QV.exe