i am not able to see the .img file after backing of the efs through this apk in the phone memory. Currently using es file explorer with root option enabled.( in Carbon 4.4.2 by xenon92). I am in doubt whether any .img file has been created or not. I have not tried the terminal emulator method though.
But using the same apk in stock 4.2.2, i was able to back up the 20 mb size .img file.
I doubt whether I wud be able to restore the same in these custom roms ....just in case it becomes necessary....