GParted is a boot CD, so its file-system is local and resides in memory, and is lost after it terminates.
You must save the screenshots on a real disk.
I quote from HOWTO backup any output from the livecd:
Open a terminal from the panel, by clicking the terminal icon and then type the following :
fdisk -ul > myfile.txt
plug your usb key and type :
fdisk -l | grep sd*
search for your just plugged key : it could be something like sda1, or if you have sata or scsi hard disk, it could be sdb1 or even sdc1... try to guess : may be you could unplug your key and run the "fdisk -l" command again to see the differences...
Imagine your key is plugged on sdb1, then type :
mkdir /tmp/usb
mount /dev/sdb1 /tmp/usb
cp
myfile.txt /tmp/usb
ls /tmp/usb
(there you MUST see myfile.txt !)
umount /tmp/usb
unplug your key and plug it on your pc (after you have booted your OS), to get the file.