What dimensions did you use for the background pic? 72 dpi I suppose? Did you find that it added a lot of load time? Can you add copy or pictures on top of this responsive background? I usially use HTML Editor (converted from dreamweaver) and not super familiar with it yet but seems like you could do the same things that you are doing.
Image has been downloaded from http://www.gratisography.com/ as I have mentioned in demo .
After download any image from mentioned above link you have to optimize the image for web as images are high resolution.
Sorry right now I am away for a week and my computer is not with me. I am using mobile phone to reply. If you go and download a pictuer from above mentioned website you will see the original dimension as I have not touched dimensions for the demo,