g5 ppc osx 10.5.8 i can't change administrator password
From the screenshot, I see that you've already gotten to "Single User Mode". (Command-S at power on should get you there unless there is an Open Firmware password - in which case you'd need to clear that by changing the installed RAM).

After mounting the filesystem:

mount -uw


You can change the password of a user:

passwd username


If you don't know the username, list the user home folders:

ls -l /Users/