To add border to pictures in WordPress Post/Page, just follow these steps

1. Appearance > Customize

2. Custom CSS (or Additional CSS)

3. Insert this CSS

p img {
border-width: 1px;
border-style: solid;
border-color: #f6f6f6;
border-radius: 5px;
}

The CSS will add border to images in All Posts, Pages in WordPress.

Border-Color: You can use Color Name (red, black,…) or Color Code (#f6f6f6, #ddd,…)

Border-Width: You can use PX

Border-Style: You can use: Solid, dotted, dashed, double

Border-Radius: You can use PX or %