With new blogger templates it's much better to uploaded backgrounds, instead of using the background codes in gadgets. I know, this might sound surprising, because I made so long long backgrounds to use with codes. I'm going to add the new backgrounds the way, that you can copy and save to image straight onto your computer, but meanwhile and with the older backgrounds this is what you can do:
You can find the the instructions to
upload the background to your blog,
instead of adding background code with gadget, by clicking "read more". 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.