Thanks. dottable has only 2 export options: Save to Camera Roll, and Post to Twitter.
However, I've now found that I can keep the transparency via a PNG file through the Camera Roll,
so it does work (I was misunderstanding how dottable did transparency)
I see that Pythonista, has an Edit option for images, but dottable does not appear here, no doubt because it doesn't handle the share interface you mention.
So what do people use that works nicely with Pythonista?