Given some of the issues folks have had with PNG lately, I thought perhaps this might be of general interest:
http://www.hanselman.com/blog/BeAwareOfDPIWithImagePNGsInWPFImagesScaleWeirdOrAreBlurry.aspx
Be Aware of DPI with Image PNGs in WPF - Images Scale Weird or are Blurry
which includes a link to:
http://brh.numbera.com/software/pnggauntlet/
I personally haven't had any troublesome PNGs lately and don't have any that need Extra Special Attention, but if
you test the above how about dropping a line back to let us know if it resolves any FM-related issues you might have had. As always ... be sure to mention your exact FM version, from Help > About, the "pxx" numbers.