You have several threads with this same question. Please refer to the multiple answers you received in those other threads.
You can manually enter the code/css:
<div class="image"><img alt="" src="../Images/sample.jpg"/></div>
div.image {margin:2em; text-indent:0; text-align:center}
div.image img {width:80%; max-width:300px}
But you must make sure your images are actually in the ePub.
Use the Sigil function to add external files, then the Sigil function to add an image to the html doc. It will do most of the work for you.