Is your layer coming from a web map? If so you'll want to configure your pop-up in the Map Viewer to add a "Media" section:
- In the pop-up configuration, click "Add content" and choose "Image"
- In the URL field of "Configure image", select the field with your URL
- Save your web map and refresh the dashboard, and the dashboard should pull in your web map's pop-up configuration
You'll get something like this in your dashboard:
