Images associated to Tweetily Posts

I have my twitter feed set up so it posts everything to face book. When tweetily posts for me the image that shows up on face book is not what I want- It seems to be fairly random. Where is this image coming from – I would have thought it would be the image used in the post that was tweeted. But it isnt.

How can I control what image is used?

http://wordpress.org/extend/plugins/tweetily-tweet-wordpress-posts-automatically/