Go to Settings>Format and paste this code into the "post template" box.
"<a href="http://www.facebook.com/sharer.php?u=THEURLOFYOURPOST"><img src="http://2.bp.blogspot.com/_XmSxWfvnwsE/Ss6YPZu8OgI/AAAAAAAAA9s/6QsHjrI2I-4/s400/share+to+facebook+button.jpg" alt="share to facebook button"></a>"
Once you publish the post. copy the URL of the post. edit the post and paste the url of the post where the code says THEURLOFYOURPOST
You will also need to adjust the template of your blog as descibed in this post
add a share this post to facebook button on your blogger page