You have to add some code to your style sheet for the class to function correctly. Some of it can be found in the default template. (oddly, they misnamed the class “aligncenter” as “centered” in the style sheet)
img.aligncenter {
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;
}
.alignright {
float: right;
}
.alignleft {
float: left
}
I guess they forgot about this change when they said it wouldn’t affect templates.