Themes: Need help to modify picture.php, the image is not centered

If you remove <style type="text/css">
td img {display: block;}
</style> from page's HTML, picture will be centered. Though you should be careful because this could affect other part of your layout, that's why you should apply this style to specific class.

(tip: use Web Developer Toolbar for Firefox, then go View Style Information, and you'll see which styles are applied to particular element; then you could see what makes problem and you should try Edit CSS or Edit HTMLfor on-fly editing)