I would suggest similar as was described by @webdeersign in your other thread.
How To: Modern Background Effect
I would add the following:
You could either make the background images a size to fit the entire Bloc including white space. Or you could just create the shapes (without excessive white space this would save on image size) and set a position in the sidebar > background > style. You can also do the same by giving each Bloc a class, then via the Class Manager assign a background image along with a position the same way in your custom class. Either way will allow you positions to choose from, so you can position your shapes as your background where you want them for each Bloc.