Backing up system

[GMS S]

Hi,
http://www.sysresccd.org/Download
With the SystemRescueCd I backed up the /dev/sda6 partition in /mnt/backup/ naming "diskimage"

After backing up operation the file has the automatic extension like "diskimage.000".
Is this file ok to restore?
If I name it "diskimage.gz" then it is automatically renamed with this extension "diskimage.gz.000"

But here below
http://www.sysresccd.org/Screenshots
the file is name like this:
"diskimage.pimg"

Do I have to name with the .pimg extension during backup to restore correctly?

And how can I restore it?