Centering images can add a special design appeal to your ebook formatting. This can be applied book-wide (to all images) or applied individually. The easiest way to center images is to make the image a block of its own and set the margins to ‘auto’.
Here is an example:
CSS
img.individual { display: block; margin-left: auto; margin-right: auto; }
HTML
<IMG class="individual" src="..." alt="...">

CSS for all images to be centered
img { display: block; margin-left: auto; margin-right: auto; }
HTML
<IMG src="..." alt="...">