Yeah, some good points for figuring out the right contrast for buttons and bubbles against the background image with different color placement. BTW, try using a gif in background. Although not always good for conversion, as it is quite distracting, but still looks pretty cool.
We are actually thinking one step ahead, something for the future, where you can change the design of the Bot in a dynamic way on a per gambit basis. So that when the conversation moves on to some other topic, the background image (and even colors) can be changed accordingly to give a visual queues.
One more design, for fitness or motivational content:
If you have some interesting designs that you have created, do share here. Would love to see them.