Image Centre Problem

I have created a custom blog theme which is looking great! I am having a problem with the images though.

I am trying to centre images on the blog, it is showing as centred on the editor in the admin panel, but when I check the post, they are back to alignment on the left. I cant figure out whats wrong, could somebody offer an opinion? Probably something in the CSS.

site is http://www.davidgreensmith.co.uk

Sample post: http://www.davidgreensmith.co.uk/2011/01/26/chichester-fashion/