Firstly, let me start off by welcoming you aboard here
RackToon!
And secondly, let me say that I love your cartoon and I thank you for sharing your obvious talents with us! [righton]
As far as changing your background is concerned, the whole key here is to create and keep your background on a separate layer within Photoshop and, in this way, you can change it "at will"!

If you are unfamilar with Photoshop's layering ability, don't hesitate to speak up and say so and one of us will be happy to quickly acquaint you with same.
In the meantime, enjoy your stay... stick around... play and learn with us all!