Tumblr help, pleaseeee!!?

hey, I need to know like the CSS name or code to make all image posts on tumblr be centered. Also, if there's a code or way to tweak the CSS to make just the post column scroll down when scrolling. I've seen those themes before where they have the about me be fixed while only the blog posts scrolls. Or if you know any nice two columned themes where the navigation links is on the top and it the about me part has the photo icon.