I've included a demo mission with an example dialog, showing a collection of 6 images, all with variations of transparent pixels and one without.
Repro Mission Download:
https://dl.dropboxusercontent.com/u/39109326/dialog_example_mission.Stratis.zip
All but the one without transparent pixels, display the same result. Every variation image has been based on the example_image1, which is an all solid image without any transparency.
It does not seem to differ how much the transparency level is. Most have 99% transparency level, some 50% or 20%. Of course, the large black box is 100% solid.