I don't know whether this will be any help. I convert raw HTML to EPUB all the time. I use Calibre only (not Sigil).
In the HTML I make sure my <img> tag is enclosed in <p>...</p> tags and that I don't have any height or width attributes set in the <img> tag. So a typical image would look like this in the HTML:
Code:
<p class="aligncentre"><img alt="c01" src="images/c01.jpg"/></p>
with this as the relevant bit of CSS:
Code:
.aligncentre {text-align:center; text-indent:0}
Edit: I wouldn't think it would matter whether your image was aligned left, right or center in the CSS.