At one point, FogBugz scaled images displayed inline and linked to the original, but our in-house FogBugz installation no longer provides that link to the unscaled image. Is there any way to get that behavior back, as sometimes the scaled image is illegible because the original is too large?
I think you might be observing a subtle difference between case event attachments vs. inline image attachments in rich text case events.
When you attach a large image to a case event via the "Attach a file" link in the lower-right corner of the editor, you will see the "linkified image" behavior you've described in your question.
If, however, you include a large image in a rich text case event via the "Add Image" toolbar button you will not see the linkified image behavior.
The following BugMonkey customization adds the "link to full-sized image" behavior to inline images in rich text case events: