Lighting image only will use the lights as specified on the Lighting Tab. User specified image uses an image to apply as a background scene for the model. Again, the lighting will be as specified on the Lighting tab.
thank you Ron, but I still have a doubt:
if the "User specified image" in the Global Illumination tab (Scene Editor) uses an image to apply as a background scene for the model, then what is the use of the image set in the "Background tab" (this also in the Scene Editor)?
The background image is just for setting your background but will have no impact on the lighting schema (jpeg,tiff and png). In contrast the "User specified image" in the Global Illumination tab (Scene Editor) uses the HDR files only for setting up the lighting schema ( mapping the multiple shades btw dark---light) for rendering the model.
Sorry my definition is very crude.