On a default installation, your Webmin login page looks like below:

To suppress both the Webmin logo and the “Webmin” text from the login page, add below to

Webmin > Webmin Configuration > Webmin Themes > Theme Extensions:

h2.form-signin-heading {
    display: none !important;
}
i.wbm-webmin {
    display: none !important;
}

The above will prevent display of both the Webmin logo and text.

To display your own logo in it’s place, go to Webmin > Webmin Configuration > Webmin Themes > Theme Logos:

Set “Logo for unauthenticated users” to yes as shown below.

Click the upload icon to upload your custom logo.

Your login page should now display your custom logo:

Note:  While possible to suppress the Webmin logo and text by editing the session_login.cgi file, you should not do so.  Any changes to this file will be over-written on any Webmin update.

Src: https://davidghedini.com/pg/entry/webmin-use-custom-logo-on-login-page/