Can't add logo to signature block in mail

I have had a company logo below my signature black for years, and now suddenly it's gone. When I go to edit the signature block there is a "missing plug-in" notification in the signature block and I am unable to drop a new version. I have tried png format, not yet jpg.

I am running Sonoma 14.1.2 on a 2022 MacBook Air.

thanks for any help!