You set image quality when you add reports to a Publish Task. There are some caveats to take note of especially as you are using PixelPerfect. See: https://help.qlik.com/en-US/nprinting/November2017/Content/ReportsDevelopment/SetImageQuality/Image-quality.htm
Here is a video tutorial: https://www.youtube.com/watch?v=DtjBI9_ANV8
Note that currently changing the image quality in the PixelPerfect Behavior section makes no difference. However, this is being worked on and will probably be functional in a later release.
HTH - Daniel.