[Koha] Patron Card Images pixelated in PDF

Hi everyone! I am new with Koha and I have been trying to print decent Patron Cards but the result is not filling out my expectations. Patron images and library logo are pixelated despite size, DPI and format (jpg png). Patron images on screen are good looking quality, but pixelated when processing batch in the resulting PDF. I have been googling around with no luck. I am running Koha 3.16 over Ubuntu 12.04 LTS (updated the system a few days ago with apt-get). Any ideas? Thank you in advance, Lisandro