What do I need in order to have custom thumbnails for posts in the wordpress.com blog

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I have a blog on WordPress.com. I would like to have it set up in such a way that when I make a post on the blog, I can select an image that I want associated with that post so that every time someone references that post on Facebook or a similar site, that individual post's selected image will appear as the thumbnail.

I tried to research myself what needs to be done – but I found the results of the web-search to be profoundly confusing. I have no idea which premium features I need to purchase (and due to budget limits I don't want to purchase more than necessary) I have no idea which theme or themes support this feature – nor do I have the faintest clue how, when authoring the post, to specify the image.

Can anyone make this information clear?

Best Answer

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: if a featured image has been uploaded for a post/page, we will recommend to Facebook that this image be used as a first priority.