Add class to WordPress image HTML elements

I need to style the element that goes around images embedded in a post
but as far as I can tell there is no way to automatically add a class to it or target <div>s with an <img> inside only without using jQuery or something.
And there is another condition that automatically add <div>HTML elements, when embedding two pictures.

<div class="sideBySide">
 <img src="#" alt="">
 <img src="#" alt="">
</div>

I have no clue and solve my problem