Your container does have a 50/50 margin. You just are not seeing it on the right because the page is much wider.
You don't want to do what you did. You are using a table to position stuff on the page. That is not the purpose of tables. What you want to do is use CSS to position things.
Here is a place you can go (for free) to learn CSS: http://www.w3schools.com/css/default.asp
I can't see where the extra 5 px on the div that contains the image is coming from.