To style images that you use in your articles, and not elsewhere, you can easily add some CSS to your template CSS options.

First off the images only concern the images inside your content area, so images you use when creating articles or blog items

Simply add the following CSS to your custom CSS option:

.item-page img {border: 1px solid #eaeaea; box-shadow: 0px 5px 20px 0px rgba(0, 0, 0, 0.6);}

inside the item-page, all images will be styled to the rules you create here. So no other images will be affected. For instance, you would not want images for a logo, or that you use in your category layout to be affected. So this makes only the images inside the item-page class change.

Check the image below: