How do I add these fish as my Tumblr background?

[blue]

I already have the code which I can use to embed it to my HTML code, but I don't know where to put it in order to make it the background:
http://abowman.com/google-modules/fish/
And I know that it can definitely be done because I have seen two blogs which have it as their background.
Any help would be greatly appreciated!