This is a two-fold problem I am experiencing, and it's a new problem that has suddenly become an issue and was not the case before.
png Images added to the main body of the StoryMap either do not display with the transparent background at the onset after Import, or they appear to display with transparent background all the way through Preview, and then change to black/glitched backgrounds at Publish.
For my project, they are either SmartArt graphics from PowerPoint (2016 version) or logos. The pngs are displaying properly in other software platforms like Adobe and between PowerPoint and Word (transparent background).
The only workaround I have found is to create an entirely new SmartArt graphic and export a new png file. Perhaps repeated edits of the SmartArt graphic in PowerPoint and export/overwrite save is corrupting the transparent background (who knows). However, even if the graphics appear as desired, as soon as I Publish it the backgrounds are black, with stripes, and so on. It's unpredictable and happens within the Sidecar as well.
Very concerned about this as the graphics are essential components of this StoryMap style presentation.