Thanks for posting your question about slecting a background image for individual pages. Unfortunately, I did not see that there is a function built into BoldGrid for accomplishing this.
However, I did notice that 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 background image to span the height (customize on each page) as an alternative.
Also, you can always submit your feedback for our developers as a featue request for them to consider. Your feedback is always appreciated. I hope this helps!