I am just running across this myself: an image used as a page header, it looks just a bit blurry when exported to PDF. This is true for either 'Save as PDF' or 'Print -> Save PDF'. This is using OSX, FMPA 15.02. For me it's not a huge difference, but certainly noticeable and makes you squint your eyes a bit.
Here's a sample that I am seeing (I was trying a couple of other solutions, that's why there are two layered images in each one).