As far as I’m aware that would be specific to the email client/service that processes the received email so if there’s some way to control the image shown it would depend of support from the mail client/service. Additionally, from the Auth0 side of things I have no knowledge of any option that could be used to try to influence it.
It’s highly likely that the current behavior you’re experiencing is Outlook and Gmail automatically generating an image based on the email address. Yours is a
no-reply@ hence they probably picked the letter
N to represent it.
If those services supported Gravatar or X-Face then you may be able to influence if from your side given that you control the email address and the SMTP provider from which emails are sent, however, this is mostly out of scope from an Auth0 perspective so I would recommend you to research the documentation for the email clients/services themselves.