Poster sizes can become quite large so in an effort to help with load times when you save a poster that is less than 6,000,000 pixels in size all the transparent features will be saved so that the next time you load the poster you won't have to refresh it. This size makes is so any collage that is less than 16" x 16" at 150 DPI will appear as you see it when you save it.
Larger posters require more resources so the transparency options are not saved. You may see black borders around your pictures. When you export your poster it will regenerate all the photos and will appear as desired.
IMPORTANT: Remember that saving is different than exporting. When you export your poster all transparency options will appear no matter what size poster you have.