mhikl-
I've seen the very same issue with image centering. My theory is that Stanza doesn't fully comply with epub specifications; though the previously mentioned reader css/priority/!important explanation is probably more accurate.
At any rate, I found that enclosing your <img /> tag with <h1></h1> tags causes Stanza to center the image. So my image references typically look like this:
<h1 class="sigilNotInTOC"><img src="../Images/ImageName.jpg" /></h1>
The "sigilNotInTOC" term keeps the image from showing up as an item in the table of contents.
Hope this helps.
-Matt