As far as I know, Sprite Backup doesn't have a default password since I have encountered the same issue with that and after contacted Sprite, I realize I have entered a password during the first time I run the setup wizard. As stated from Sprite, they didn't put any password for the image file. It is not possible for Sprite Backup to ask for the password if the user didn't enter a password.
I suggest you contact Sprite support to ask for the help and also I suggest you try to remember if you have ever entered a password when you run the setup wizard after you installed Sprite Backup.
Good luck
