I never had this problem with version 1.72 and below. However, the new versions (including 1.81) no longer show thumbnail images on my PS3 for mp4 files (or any files for that matter). Instead, all I see are large arrow buttons instead of images.
Note, I have never changed the default settings (the default settings are checked for "Generate thumbnails" and "Image thumbnails").
I am running PMS on a Mac Pro running OS-X version 10.6.8
Anybody else having this problem? Please help!