Hello,
I had a similar issue, all my banners were just plain blank. Strange think was that even then I put direct url to image, all browsers ditn’t show images and said that images were 1×1 size on laptops , but everythink was ok on desktop computers. So I just created new folder on my ftp server and copied all images, then changed urls in bannerize settings and everythink came back to normal. I don’t know if that was bannerize problem or just conflict with other plugins. I’m using wp 3.5.1 and wp bannerize 3.0.62.