You will have to use CSS to just add the border on the left or right side. Will simply have to assign the image a class when adding within a post. There is a field for the class
For example you could create a css class called customclassborderright or customclassborderleft and then put that in the css class field in the post editor after adding an image.
Of course you will need add this class to your custom.css or style.css sheet. Using the examples above you would add this value for example:
.customclassborderright { border-right: 1px solid black; }
or
.customclassborderleft { border-left: 1px solid black; }
etc.
Good luck!