This article explains how to edit table and image borders when a link is attached.
Change Border Colour
Selecting the TABLE breadcrumb will be present you with the option to change the border colour.
You will need to set a border width in order for the border to appear.
Change Border Width
If a blue border is appearing around images with links attached, this is because certain email clients will automatically insert a blue border around the image.
To remove this, change the border width to “0”, this will then remove the border from the image.
You can see here that the Border Colour has been changed to red. This is reflected at the options underneath the breadcrumb where the drop icon is red.