I tried the same with GIMP, and it works too.
I think you have to copy a single layered picture. I just opened a PNG picture in GIMP, Ctrl-C, Ctrl-V in CC's image editor, it worked.
I opened a multi layer xcf, ctrl-c (one layer was selected), Ctrl-v in CC's image editor, it copied this single layer too.
Beware of layers.
It works under windows XP at least.