Hi there!

Thanks for your query.

You can use the following code in your custom CSS to change the image and icon there:

#login .login-bg-img {
    background-image: url(../images/login-sidebar-img.jpg);
}
.fn-man-user:before {
    content: "\e931";
}

Please use your desired image link in the above code, you have to upload that image in your media before that. You have to use content code.

The following doc link may helpfull too:

https://www.joomshaper.com/documentation/joomla-templates/finion/icons-package

Note: Path towards custom CSS(Joomla 3): Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.

Best Regards