Image doesn't display in email signature in Mac Mail App when sent from iOS15 Mail App

I picked up my new iPhone 13 Pro running iOS15 last week.

I've installed my HTML email signature that worked fine (and continues to work fine) on my old iPhone 12 Pro running iOS 14.8.

If I send out an email with my signature from my new phone, the signature displays fine in all email clients (Hotmail/Outlook, Gmail, Outlook for Mac) except Mac Mail App (running on M1 MacBook Pro with latest Big Sur 11.6). In Mail App on Mac the signature disappears.

If anyone can help with a fix I'd be mighty appreciative! Otherwise I may have to return the new phone to Apple 😟