There are 6 total thumbnails that an image can be resized to. Each one is accessable by appending a single character suffix to the end of the image ID, and before the file extension.
For example, the image located at http://i.imgur.com/12345.jpg has the Medium Thumbnail located at http://i.imgur.com/12345m.jpg
The thumbnails are:
s = Small Square (90x90)...
Had you said previews on the progress bar (or even had uploaded images) I might not have misunderstood first time around. IF that is what you mean:
The progress bar preview works on some videos and doesn't work on others. There is no explanation for it, nor can you fix it.
from YouTube Help Forum might be of interest (though I'm not convinced correct).
There is a inbuilt functionality called featured images in WordPress:
Reference : http://en.support.wordpress.com/featured-images/
I am not sure whether it will be displayed in Facebook Embed Post but it is worth giving a try if you do not already have.
See the below reference: