You can access the link on all public assets by opening the asset and choosing Share Asset. Under the dropdown you'll see an option to view the Embed link.
You can place this link in an image tag like <img src="link_here"> and when you update the asset, it will update the link wherever it lives. If an asset has multiple attachments, whatever attachment is in the top position will be displayed using this link.
Be careful not to delete the asset when using this feature!