Images exported (shared) from an ArcGIS Pro Layout should honor the background transparency setting. Currently when the layout is exported as a PNG with a transparent background it defaults to white. In ArcMap this is not an issue as you can set the background and transparent color. This question was originally raised in July 2016 in thread Exporting PNG image with background transparency from ArcGIS Pro .
This would be a great enhancement and improvement to the sharing feature in Pro.