When uploading videos to the composer, Buffer will pull thumbnails from 15 second intervals of the video. The thumbnail image you choose will only be applicable for social networks that don’t autoplay the video, which include LinkedIn, Pinterest and Twitter if the length of the video exceeds 140 seconds. Although Google+ does not autoplay videos, unfortunately custom thumbnail images are not supported and therefore the first frame of the video will be used.
For other social networks including Facebook, Instagram and Twitter if the length of the video is within 140 seconds, the thumbnail will be disregarded because the video autoplays and therefore starts on the first frame.
We're really sorry this isn't a very clear process. One of the tricky parts is that as you're creating your post, we don't always know which social networks the video is going to be shared to, which is why we display the option to select a thumbnail for all of them. We'd love to find a way to make this smoother.