voon, many of us use bg images for setting up scenes only, but do not actually render bg as part of the final render (render with alpha), as we like to comp stuff in PS and have more control by keeping stuff separated. If you have alpha enabled in render target, you can then see your bg image (jpg etc). If you have alpha disabled and do not have a bg image selected, the background you would be seeing is either the hdri you have plugged in or the daylight system. And when alpha is enabled and bg image is selected, you see bg image. So yes, you can have a HDRI for lighting and bg image for comping and setting things up etc. I don't have Octane open at the moment, but I'm 100% certain that bg image has zero effect on lighting, reflections etc.
So go ahead and enable alpha in rt, plug in a hdri into either daylight or texture environment and take this mofo for a spin.. play around get yer hands dirty. And bone up on HDRIs bro... a half hour or an hour learning about how/why HDRIs work would be time very well spent.
