Image Alignment Problem in Single post page

Hi All,

Before 2 days i changed the theme of my blog, after changing the theme the images in the single post page are always getting left aligned, if i see the images in the post editor they are aligned correctly, i already googled about this issue and found adding img.aligncenter , img.alignleft, etc …. to the css will fix this issue, i added that to my css but it didnt fix the problem, when i see the source of my blog i am getting the img tag like this <img class="aligncenter size-full wp-image-811" src="http://www.mydomain.com/wp-content/uploads/2009/07/logo-small.gif" alt="logo-small" width="116" height="134" /> is this right ? please help me to solve this issue, my full blog posts are not good because of the image alignment problem.