So one of our devs go back to me on this.
There is a bit of a tweak some need to make due to transparency "stuff" going on.
Here is how you can get this to work consistently no matter how your have your capture or Editor options configured:
1. In Editor choose the Image tab.
2. Select Modify group > Color Effects > Color Depth
3. Change the color depth from 32-Bit to 24-Bit and close the dialog box.
Pasting the image into Gmail should now work.
I hope this helps solve the mystery.