Parallax Speed

Is there any way of slowing the speed of the parallax background image? Currently the images can be seen repeating underneath. I’m using the following code…

[su_section border="none" image="http://www.phenixdigital.co.uk/brookeprice/wp-content/uploads/2015/02/newscroll.png" padding="120px 0px" text_shadow="none"]
[su_panel shadow="none" border="none" background="rgba(0,0,0,0.7)" color="#ffffff"]Lorem ipsum dolor sit amet, consectetur adipiscing elit. Proin at sagittis lectus. In hac habitasse platea dictumst. Ut facilisis imperdiet dolor non rhoncus. Sed posuere nisi vitae hendrerit tincidunt. Ut felis sapien, lobortis vel risus eget, pharetra ultricies nibh. Nullam nisl erat, bibendum porta orci ut, scelerisque bibendum mauris. Suspendisse egestas, leo semper vulputate suscipit, libero tellus sodales nibh, quis fermentum dui urna vitae nulla. Vivamus lacinia, eros at condimentum molestie, turpis lectus faucibus felis, quis venenatis velit eros vitae lacus.[/su_panel]

https://wordpress.org/plugins/shortcodes-ultimate/