What may have happened is that the background color of the page was originally set to solid white (although it is now a gradient). The balloons are also white. So what happens is the white in the image is made transparent, since it matches the original white of the background.

Here's the fix: Open the Background Color dialog, change the background color of that page to solid red (or any color you prefer), and click OK. Open the Background Color dialog again, change it back to gradient and click OK. The balloons should not be transparent anymore when you preview the site.