Here are the instructions to upload the background to your blog, instead of adding background code with gadget. You can make this change and still use the same background you have chosen. This needs only a little bit effort, but the result is worthy.
1. First step might sound odd, because you need to add the code of your chosen background to the gadget. After done that right click > View Background Image > Save Image As... (you need to remember this location later). Then remove the background gadget.
4. Click Background (on the left) > click Background Image > Upload image (Browse the image you saved onto your computer). Push Done.
5. Set the Alignment up middle and uncheck the Scroll with page.
6. Adjust the Entire width (with my 3 column backgrounds 1000 px, with my 2 column backgrounds 800 px). Apply to Blog. Sidebar wights you can check later depending the next step.
7. Choose the layout. I have chosen the 2 column layout, even thought the background is for 3 column. I like the wider center. But you can surely choose any layout you like. Apply to Blog. And then go back to step 6. and adjust the wight(s) of the sidebar(s). Remember always Apply to Blog.
So far so good. You find the next step here.