PNG/TIF low quality (Mapfish)

Hi, I'm using Jasper reports with Mapfish. Printing PDF is ok. But when I'm trying to print PNG or TIF, output quality is low and images are blurry. Is it possible to change DPI of output? I've already tried to change net.sf.jasperreports.image.dpi, but not sure if in right place, because it didn't make any change. Thanks for any help