Thanks for posting your question about selecting a different background image for each page and section. Unfortunately, that function is not built into BoldGrid. However, you have the option to change the background image to a pattern or solid color, to remain universal among the pages. Then, you can add a GridBlock, select a background image, and expand the height of that section to fit the page as an alternative.
Also, you can always submit your feedback for our Developers as a feature request for them to consider. Your feedback is always appreciated. I hope this helps!