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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)...


2

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).


2

<meta property="twitter:image" content="<URL-HERE>" /> You can use this meta tag for twitter. For more informations see here: https://dev.twitter.com/cards/markup


1

It might probably be the format in which the video was uploaded. For instance where the script used in handling the thumbnail function can only read certain formats of videos like mp4


1

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. EDIT: See the below reference: http://en.support.wordpress.com/facebook-image-thumbnail/ Featured image:...


1

Check out Remove Image Search Zoom Extension. Full Disclosure: I wrote this after reading your post.


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