This is really easy*. But the version of the OS doesn't matter. it's the version of iPhoto that does.
So, if you have iPhoto 6 or earlier:
Go to your Pictures Folder and find the iPhoto Library Folder there
iPhoto 08 or later:
Go to your Pictures Folder and find the iPhoto Library icon there
In either case then drag the iPhoto Library from the Pictures Folder to another Disk. The Disk should be formatted Mac OS Extended (Journaled).
Once that's done test the back up - open it and view the Photos: To choose a Library to open: Hold down the option (or alt) key key and launch iPhoto. From the resulting menu select 'Choose Library'
Assuming the Library is working okay that's the back up part.
Then the restore:
Drag the Library from the External back to the Pictures Folder. Then Hold down the option (or alt) key key and launch iPhoto. From the resulting menu select 'Choose Library' and point iPhoto at the Library in Pictures.
*Easy that is unless you have a referenced Library. A Referenced Library is when iPhoto is NOT copying the files into the iPhoto Library when importing because you made a change at iPhoto -> Preferences -> Advanced. (You unchecked the option to copy files into the Library on import) The files are then stored where ever you put them and not in the Library package. In this scenario you are responsible for the File Management.
IF you have a Referenced Library then you have a much more complex job. You'll need to convert it to a Managed one, and the do the steps above.