The <center> tag is a depreciated tag in html and shouldn't be used. In order to center the items on that page, you will need to add align="center" to the <table> tag, as well as any <td> tags just to be safe. You will also need to edit the buy_now.tpl the same way to get the quantity line and the buy now button to align properly.
If you want to center just a single item in a table without centering the rest, try enclosing the item with div tags. Example:
<div align="center">Item you want to center</div>
That should solve it!