You can find the thumbnails in <User>\AppData\Roaming\SKYBOX\data\thumbnails.
Means you can change the thumbnail manually, but they have cryptic names and if you remove and add videos from your list it won't refresh the edited thumbnail and you have to start from the beginning. So once u edited it, don't touch it.
But with the current videolist thumbnails aren't the big deal at all. They show 6 videos/thumbnails at the same time, but they are small and not really attractive, even if you have the custom thumbnail. You can move the VR position further forward, which would improve the situation a bit, but atm the videolist is more functional instead of attractive.
Netflix e.g. have an excellent system for a video preview:
You still have a list where you can scroll through at the bottom but also have a big screen (50-60%) with Info, Ratings and a good big Thumbnail, on top after some seconds a trailer starts... nice!
Prime Video from Amazon also has a rather functional design. It works because people have a prime account and watch PrimeVideo on TV for free movies/series and if they buy some videos, they almost know what they buying and not because of the trailer.
For me SkyboxVR is most wanted player in GearVR/Oculus Store and have the potencial to get a big name in the VR World. Samsung announced last month the "Anti SDE AMOLED" campaign with 1200ppi Displays (~570ppi with Samsung Galaxy S7+). While HTC Vive and PSVR focus on gaming, Samsung seems to capture the entertainment sector. That means in the near future you can watch movies with good quality/resolution with the next generation GearVR. inclouding already existing 3D-Movies and new kind of content.
SkyboxVR have the potential to get the big player on this marked and hopefully his app will pay off for the SkyboxVR-Team. But I think a big step in that direction is the presentation of the Videos, like good thumbnails etc.