How can I add row level security in with subscription using images preventing email forwarding?

From what I understand this requirement is that the subscription is generally based on whoever sets it up so there is no RLS applied.

The picture is also a screenshot taken of a specific page which obviously has no extra protection.

The solution that amitchandak posted regarding Power Automate could help you but you need to set it up for every recipient manually from what I saw?

Alternatively, and this is not a good solution, you could use Paginated Reports which now support dynamic subscriptions (https://powerbi.microsoft.com/en-us/blog/dynamic-subscriptions-for-paginated-reports-preview/).