Change the default WordPress image on the dashboard login to a custom image [duplicate]

You can upload an image to your theme and add this to your functions:

add_action("login_head", "my_login_head");
function my_login_head() {
    echo "
    <style>
    body.login #login h1 a {
        background: url('".get_bloginfo('template_url')."/path to your image/logo-login.gif') no-repeat scroll center top transparent;
        height: 90px;
        width: 400px;
    }
    </style>
    ";
}

Source: https://wordpress.org/support/topic/change-login-picture