Since you didn't mention which burning software using, it's hard to know exactly where the current background templates are, but if you find the current backgrounds, check to see their "Properties" and write down their resolutions. (Maybe 768 x 576).

Once that's done, take any picture of your choice, resize it to the correct size using a program like the free Irfanview, then copy the resized picture to the same folder as the others..(With some burning software, you'll need to "import" the new picture using the program GUI instead of simply copying it to the storage folder.)

Hope this helps.