Online Documentation for Advanced Data Export .NET
public string BackgroundFileName
The BackgroundFileName property sets the background image of the result HTML document. If an empty string or not existing file name is specified as a property value, the document won't have a background image. When setting this property, it is recommended to specify a relative file path (for example, ./Img/html.jpg), that will let you change the location of your documents without making any changes to the HTML code. It is also recommended to select the background image of the page close to color scheme to the background of the document (see BackgroundColor), otherwise the users that have the image display mode turned off can get not a very correct image of your design.