After you place your image tag, please go to Admin->Help Desk Productivity->Automations->Ticket Updates (formerly called Observer):
- Click on New Rule and name the rule: Unread Ticket Tag.
- Under When an action performed by select Agent & Any.
- Under Involves any of these events add Reply is sent.
- Under on tickets with these properties: Don't change.
- Under perform these actions: Add tag and name it unread.
- Click the Save button.
Here is an image of how your Observer Rule should look: