Apple Mail Signature Issue: Missing-plugin When Placing Image in Mail Signature

To add a signature to your email on Mac, you go to mail application, press mail on top left then settings:

You need first to make sure that composing is set to "Rich Text" mode:

Then click on signatures tab, and click the plus button:

Then drag and drop your image from finder:

Do not forget when composing to choose the signature if its not set as default: