When trying the load a shapefile as a background file, the following error message is generated:
"Object reference not set to an instance of an object"
The likely cause of the issue is that there is some missing geometry in the shapefile that is causing the error message, and thus not loading the background properly.
Open ArcGIS and run the tool Repair Geometry (found in the ArcGIS toolbox). This should fix any issue with the shapefile. Once completed, try reloading the shapefile as a background file. The error message should no longer occur.
Long time to load background layers
Background layer causes program to crash
Out of memory error when loading a background file
Background image quality poor on some computers