1. This would break accessibility requirements.
2. Scammers can fake PNGs just like HTML.
2. Scammers can fake PNGs just like HTML.
Edit: Just tried it with Chase, Merrill Lynch, Citibank, Bank of America, and Wells Fargo. Only Wells Fargo had a CSP in place to prevent this. But even Walls Fargo let you just inject a data URL image.
They are not in any way interested inn tightening of fortgaurding their portal's rendering, until it ends up causing them to give more money i.e. bad for business.