if these don't already exist in your theme's stylesheet, create
To make it float right,
In the post editor, edit your post and go into the HTML tab. For the post you want to float right, find the <img tag
<img class="alignright" src="... "
If it already has a class for alignleft or something similar, remove that and leave the alignright. That will float it right.
If you want to center it, an easy way is wrap it in a div
<div align="center"><img ... /></div>