You need to include the WP default image handling stuff in your css.
When you assign center/left/right to the image, it drops the class onto the image, but if you have nothing in your css, there is nothing telling it what to do.
here's my image stuff, you'll need something similar....
img.centered {
display: block;
margin-left: auto;
margin-right: auto;
}
img.alignright {
padding: 4px;
margin: 0 0 2px 7px;
display: inline;
}
img.alignleft {
padding: 4px;
margin: 0 7px 2px 0;
display: inline;
}
.alignleft {
float:left;
padding:2px 0 0 10px;
}
.alignright {
float:right;
padding:2px 10px 0 5px;
}
.wp-caption.alignleft {
float:left;
margin-left:0;
margin-right:5px;
}
.wp-caption.aligncenter {
margin-left:auto;
margin-right:auto;
}
.wp-caption.alignright {
float:right;
margin-left:5px;
margin-right:0;
}