So here's what I have noticed since my previous post:
If you screen cap your selection, you need to go to the EDIT button at the top-left of your screen. In the drop-down menu, select COPY IMAGE. You can then paste in MAIL and in iMessage.
However, what I have noticed is if you screen cap and attempt to DRAG into MAIL/iMessage, this will not work. Further, if you attempt to COPY IMAGE from drop-down menu, after having attempted to DRAG, it will also not work. You will need to capture again followed by the EDIT / COPY IMAGE / PASTE (into Mail or iMessage).
Hope this helps some of