There is an option to add a default background image. Go to “Appearance-Theme options” Scroll downs a bit and you’ll find a field to upload images for your background slider. Make sure to click on the save button for the changes to take effect.
Every page can have its own background slider. Simply open the page you want to add a background slider to. Underneath the input field for the content of the page you will find a box called “Page Extra”. Here you can upload your background images (“Page Extra” not showing? On the top right of the page – click “Screen Options” – check “Page Extras”)