Giving the CSS of
background #FFFFFF url('images/newbackground.gif');
is just giving an example based on basic CSS. Be careful when copying the suggestions made in the book because you missed an important colon after background. It should be
background: #FFFFFF url('images/newbackground.gif');
Also, you need to make sure the URL part of the CSS is correct for the file you uploaded.
One suggestion can be that while you can do as suggested by the author of your book, it may be more beneficial (and more intuitive) to use the Media Library facility within WordPress rather than uploading directly into the twentyseventeen image folder using FileZilla or other FTP program.
This way you can link to the file using the information provided within the Media Library regarding the image you wish to use.
Take for example the following image in the WordPress Media Library of a website I maintain.
If you look to the bottom right of the screenshot, you will see the image file’s URL and a Copy URL button so the Media Library helps you by providing the information for the url('images/newbackground.gif') part of the CSS. In this case it would be url('/wp-content/uploads/2019/05/articles-cover.jpg') as you don’t need to include the domain part of the URL.