To do this, you need to edit the style in question, then select Content Types in the drop down displayed below:
  1. Next, click the Photos Content Type (Icon that looks like a Camera) 
User-added image
  1. Select the Arrows check box under Display                                    
User-added image
  1. Click on the Image icon   (Gray Image Placeholder)                       
User-added image
  1. Click on the dropdown options and select Prev Arrow and or Next Arrow
User-added image
  1. Click the Dropdown for Background Image "Please Select"                        
User-added image
  1. Select + Create New  (or if you already have a image created you can select it)                         
User-added image
  1. Add your Image from Computer and Save & Close
User-added image
  1. From here you will select the Image in the Please Select Background Image dropdown (as shown in step 5) 
  2. Save the Style