Mtpaint snapshot (default screen capture that you can find in GRAPHICS - MTPAINT SNAPSHOT) is waiting a default time of 13 seconds to take a photo, so you have got enough time to adjust your screen.
It is possible it is excesive time for you, so you can adjust it changing the default values in /usr/bin/mtpaintsnapshot.sh
I modified it to 7 seconds, capt included.
Adjust it the way u want.-
Have a nice day u all