To add backgrounds and watermarks to uploaded images, set the path in the 
background/watermark attribute of the image element in Web.Backload.config
Background images are only used in resize modes with canvas (fit, place)


Note: Background and watermark images are a Pro/Enterprise features, 
while background color is a Standard feature




Example:

<backload xsi:noNamespaceSchemaLocation="Config\Web.Backload.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:name="urn:backload-schema">

  <!-- Sets a resize mode, background, watermark and vertical watermark position -->
  <images resizeMode="fit" background="~/Backload/Imaging/background.png" watermark="~/Backload/Imaging/watermark.png" watermarkPosition="bottom" />

</backload>