The email signature preview OK until you activated campaign tracking. 

Then some images preview as white icons on a white background - They look as if they disappeared:


Vector graphics rendering (VML) doesn't support transparent background and we don't have any workaround.

It is all on Microsoft's end. In the above example the images are still present, not missing. But they are white icons on white background.


Avoid using transparent images when campaign tracking is activated.

Instead create images/icons with white background.