REGISTER TO REMOVE ADS
into the CSSbackground-image:url(url to image here);
Lol i only now just noticed that you had a link to your tumblr in you OP... (damn i must have been really tired yesterday...
^^^Pretty much this...^^^If you want to change your sidebar completely then you'll either have to get a new theme or get a web designer to create a custom sidebar for you. As far as adding additional content to I think that's something on your end.