Scrolling Background And Whiteness?

I just started using Beta 3 today. I've loaded up an 800x600 image and have set it to scroll vertically. However, it scrolls, and is then followed by a blank white screen the same size. Is there something I have to do in the 'repeat' option to not have this white background? What I'd like to do is either have the same image looping seamlessly, or to let it alternate with other background images, also seamlessly.

Can anyone help point me in the right direction?