Video Poster Image Sizes

We're developing a course which includes several videos. These videos transition from slide to slide. Unfortunately, some of the transitions end up having this weird popping blur effect:

We did some investigating and it looks like Storyline randomly changes the size of some poster images to a much smaller size. As these videos are in HD, the majority of the poster images are correct. However, you can see that many end up being 108 x 38 instead, leading to this pixelated blurry transition:

Deleting and reimporting the videos does not fix the issue. Right clicking on the video file in SL3 and selecting "Set Poster Frame" does not fix the issue. And considering that the 108 x 38 thumbnails are so small and pixelated, it is impossible to determine what image they should be manually replaced with. Can anyone tell me which document in the published output (html file, xml file, etc) would reference which slide each of these poster images appears on? That would at least speed up the process of figuring out which images matches with each slide.

Or is there another solution we should be aware of?