In Main Options > Content Areas you can set image borders. But that applies to the whole site. When that option is left unchecked, how can you add a border to images on a specific page? I would like to do that on this page: http://www.nordoy.net/folkeliv2/
If you globally want no border and only want it on a few images, add it as an inline style in the image tag.
After you insert the image, switch your content editor to text mode, and add the inline style like below. There are various CSS property that can be used to create a border effect, you will have to choose what you want.
To simply add a colored border you can use border:1px solid black; type of CSS property. You can also add a Box shadow, and you can eventually add padding if you want a double border, like a large white border with a thin darker one on the edge.