To ensure that your design changes take effect and are visible to your visitors, flush your Drupal cache after changing your background image, adjusting theme settings (colors, fonts, etc.) or using CSS Injector.
To do this, go to your admin menu at the top of the screen. Go to Configuration -> Development -> Performance. Click on the Clear All Caches button. Wait a few moments for the system to complete its task. You'll see a confirmation message.
To verify that your design changes are working, view your site in a separate browser, making sure to preface the address with http:// instead of https:// so you can see what general visitors will see (as opposed to logged-in content managers).