You can expose the image through a site. You only need to change the instance url by the site home page.

Also, you need to establish the article as public.

Site Profile Changes:

- Set field where image is placed as visible.

- Set Read permission to object where image is placed.

- Go to "Category Group Visibility Settings" section and set visibility to "All Categories" in the article/s group/s.


For example:

If you have this site:
> https://mysite-developer-edition.cs20.force.com/home

And your original image url is: 
> https://c.cs20.content.force.com/servlet/rtaImage?eid=ka3m0000000XXXX&feoid=00Nm0000000XXXX&refid=0EMm0000000XXXX

Your public image would be:

> https://mysite-developer-edition.cs20.force.com/home/servlet/rtaImage?eid=ka3m0000000XXXX&feoid=00Nm0000000XXXX&refid=0EMm0000000XXXX

This workarround would work with any image in a Rich Text Area.