By default, images don't have any borders:
<p> <img src="lotus.jpg" /> </p>
But if you want they can be framed (default functionality in the article editor):
<p> <img class="fr-bordered" src="lotus.jpg" /> </p>
Or they can have shadow:
<p> <img class="image-with-shadow" src="lotus.jpg" /> </p>
Or they can even have overlay:
<p class="image-overlay"> <img src="lotus.jpg" /> </p>