Originally Posted by Tact

I don't see the option either. They might have did away with it and expect that type of style to be changed via your theme's style.css file. If you want to apply the same border size, color, etc., to every image you post automatically, via css, use the inspect element feature of your browser to determine the proper place to add:

border: 2px solid black;

Changing the size, type, and color to suit your needs, of course.

My current theme's css element to change just post images looks like this:

#main-content img {
max-width: 100%;
height: auto;
border: 2px solid black;
}