Hi Bob,
Are you using a link to the Master digital object of the image linked to your description, or the reference copy?
If you're using the master link, then remember that by default public users can't see Master digital objects, which may explain why it's not working - the browser in this case can't authenticate and get access to the master.
Depending on how you're doing it, it may work if you go to the reference display copy (i.e. the version displayed on the view page of a description), and user your browser's right-click "View image" option to open the reference display copy in a new tab, and use the URL for that.
Remember as well that if the description is Draft, the same problem applies - so your description will need to be published!
In terms of using Markdown to create the link as Surya suggested, we have some guidance in the documentation here:
Let us know if this helps!