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:
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.