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.
<img style=”padding:5px;border:1px solid grey;” class=… src=….. />
If you are going to need to do that a lot, you could create your own class to hold the styling, and simply add the class to the image tag.
For example to create an image border class, you would add the rule below in Main Options > Fonts & Custom > Custom CSS Rule box
.myborder1 {padding:5px;border:1px solid grey;}
Then to apply the style to your image, you would just add the class in the class attribute of the image (with a space between classes). It would look like
<img class=”myborder1 otherclasses…” src=….. />