There are sporadic issues with generating patterns with some images. I have not been able to find the issue, but if you would be so kind to send one of the images to the email address firstname.lastname@example.org it will help me to debug the problem. It would also be great when you send the used settings, i.e. brand, size etc.
In the mean time, can you try setting the PDF colors setting to Black and White? I suspect the problem to be in the color generation code which has been added relatively recently. The black and white generation software has been longer around.