When using ctrl-A (Select All) the underlying original image should also be selected, not just the additions (like shapes, arrows etc).
This then means you can use ctrl-C to copy everything. I know there is Ctrl-Shift-C but I think it is more natural to use Ctrl-C.
It makes learning and using this tool a lot easier:
1) Take a screen grab
2) Edit it
3) Ctrl A + Ctrl C and paste into your tool of choice (if you cannot use the options you have provided)
While Ctrl-Shift-C is useful, I think the usability expectations of Ctrl-A is that All means image as well. This then makes copy (ctrl-c) just copy whatever is selected, potentially removing the need for the separate command, Ctrl-Shift-C,
This is how many other such tool, e.g. SnagIt, does it.
Btw, very nice tool though! Only been using it for about 15 minutes and like it and may uninstall SnagIt in favour of this, especially if I can use Ctrl-A, Ctrl-C as per above :)