How to change the Craft 3 activation email? In my organization many of the employees believe the user activation email sent out by Craft is spam.  How do I change the email text and possibly add an image, like our company logo, to the email?
Under Settings > Email there is a field called "HTML Email Template" which you can use to point to a single template that handles all of the system emails. It's not very obvious from the Craft UI, but you can use that single template as a routing mechanism for including per-email templates using the variable emailKey which is available in the template. Your routing template could look like this:

{# Craft has four predefined email keys:

    account_activation
    verify_new_email
    forgot_password
    test_email

... route to them here, or fallback to default
#}

{% include [
    '_emails/' ~ emailKey ?? null,
    '_emails/default'
] %}


You'd then have another template called account_activation.twig where you can go wild with whatever modifications you like. Each email also has a link variable available in the template to output the link to whatever it is.