Can I add custom thumbnails?
Yes, you may add one or two custom thumbnails to be used instead of Meta's auto-generated thumbnails, as follows:
Video feed
Video story
Thumbnail feed
Thumbnail story,
Or:
Video (single)
Thumbnail
How to add thumbnails?
When you select "Video" as the Ad Format, you’ll first add the main video assets you want to use for the ad. Then, by clicking the “+” button in the asset column, you can add up to two images, which will serve as the thumbnails.