Cannot change background image on posts page

Hello

I am using the analytical lite theme and I am wanting to change the background to an image on my blog ( bass clef). I have tried everything under the sun and it will not change. I have tried to upload my image as the background image on the page, no luck! I have tried to manually type in the code of changing the background with the page-id, no luck.

I do not know what else to do. I have deleted the theme and re-installed three times, and the theme does not activate with its original presets. I have deleted the cache, its as if the theme isnt responding to its own functions. Any assistance will be greatly appreciated.

http://www.elyssamusiconline.com/bass-clef