Adding a Frame to all Images Within CSS

I write a lot of documentation with screen captures, but many times the images have white backgrounds and don’t like without a border.
You can add borders in Photoshop, but if you have access to the CSS, it may be easer to write a style to add borders around an image.
A simple example is

CSS - img.border {border: 1px solid black; margin: 7px}
HTML - <img src="" alt="" class="border" />

Adding the margin also ensures there is a nice cushion around each graphic.