Now you have got your layout. You will have two options to customize the background.

Option 1: Use a photo.
Click " Search">> "Free Photos" and you could see many photos here. Double click to choose one.

If you have your own images, click " Upload" >> "Upload your images ".

After choosing your image, you could edit it in anyway you wish to: crop, filter, adjust brightness, move forward backward, duplicate and even delete.

Option 2: Use DesignBold's background library
You could go to "Background" tab and choose a ready made background. Design Bold library offers various types of background design including textures, patterns, large images and solid colors.