Did you set the image display to block? If not, changing the margins won't fix it. If you want to center all images in your passages, you can put this in your stylesheet:
.passage img {
display: block;
max-width: 100%;
height: auto;
margin: 0 auto;
}
Otherwise, if you only want to center certain images, you can wrap them in a div and create a class for it with the same settings:
.passage .centered-image img {
display: block;
max-width: 100%;
height: auto;
margin: 0 auto;
}
Then use it in your passage like this:
<div class="centered-image">
[img[image_name.jpg]]
</div>