My client has been struggling with print quality ever since the move to 12 with logos which began printing VERY poorly in FM12. These logos are not on the layout but switch based on factors and are stored as containers.
I found that switching to PNG increased the quality to fuzzy from blocky when there were JPG
I then found that if I TURNED OFF in containers setup to create thumbnails everything was great again.
From my tests it appears that the container always returns the right sized thumbnail for screen viewing but when printing FAILED to send a thumbnail to match the print quality. Since this cannot be controlled on a field level I had to force all thumbnail OFF until Filemaker can either fix which quality thumbnail is sent or allow on the field level to restrict the use of thubnails.
Oddly enough, if you pasted directly on the layout the same logo file next to the field based version the field based version would step up and print right.
Attached is an example of the three version of the logo. ALL logos are the same size. The work is a JPG, the fuzzy is PNG and the clear one is with thumbails off. The lower pair of logos represent the same PNG fuzzy logo printing right when a physical copy is also on the layout.