I love the idea of bubbles. Who says you need an example? Just take your choice of quilt marker, and draw circles where you want them. Then free-motion them.
When I made my son's quilt (planes in formation), I quilted wavy lines down the length of the quilt to look like wind over their wings. Then I free motioned some clouds around the edge. I am sure the quality is severly lacking (it was my first attempt at free motion), but I would not change the way I quilted it, even now. I love the finished project.
I would start out with smaller ones down low, then get gradually bigger as you get towards the top. Isn't that the way bubbles do in water? Just my input.
Robbi