[Plugin: Facebook] photo to represent my blog when using like button

hello,

when i click ‘like’ on a blog post, i am given an option to comment etc, but somehow the photo that represents my site has been chosen by unknown means and is from an old post!?

how can i tell this plugin what photo i want to be used to represent my blog (or maybe individual blog post)? every post, when i try to like it ,gives me this same image.

thanks, david.

http://wordpress.org/extend/plugins/facebook/