Hi @MOKRA: There is a CSS file on the backend that can be updated. You just need to add something like the following to the stylesheet.

img {

  border: 1px solid #000;

}

See How to change the CSS for Knowledge Articles.

Then, alternatively,  if you have an image in an article where you don't want a border, you just place "0" in the Insert/Edit image > Advanced > Border width field.

Hope this helps!