Use margin:0 auto;
If you’re centering something else, margin: 0 auto; will get your element centered most of the time. (Quick note: your element must have a declared width for this to work.)
The margin: 0 auto; rule is shorthand for 0 top and bottom margin, and automatic left and right margins. Automatic left and right margins work together to push the element into the center of its container.
Youtube video of katytastic in action!
The Self Publishing Podcast “Ask Us Anything” hangout #1
example of embedded bookcover from Flickr with add media install from url: