Could Hintergrund also refer to the background of a person (history, family, social class etc.) as in English, or only to a physical background (landscape, decorations etc.) ?
Both is possible; der persönliche Hintergrund as well as die Kirche im Hintergrund and sie agieren im Hintergrund.
Can it mean the background picture on the computer
It could, though Bildschirmhintergrund (screen background) or Hintergrundbild (background picture) are probably more common.
It can even mean something like "reason, motives" if you are looking at an act.
"Background" was marked as incorrect. It wanted me to use "backdrop" instead. Any reaaon "background" would be wrong?
Could der here be seen as a translation of his given context? What is his background