There’s no such thing as float: center; You can choose either left, right, or none.
http://jsfiddle.net/vd7X8/1/
If you float: left; on the image it will do what you’re after.
If you want it centered, then you’re going to have to wrap the image and the text in a container, fix the width of the container and do margin: 0 auto; on it, then continue to have your image floated–except it will be constrained by the wrapper.