By default, they are stored in your iPhoto Library, and that will be in your Pictures Folder, unless you moved it.
Some FYI:
For help accessing your photos in iPhoto see this user tip:
https://discussions.apple.com/docs/DOC-4491
Most Simple Back Up:
Drag the iPhoto Library from your Pictures Folder to another Disk. This will make a copy on that disk.
Slightly more complex: Use an app that will do incremental back ups. This is a very good way to work. The first time you run the back up the app will make a complete copy of the Library. Thereafter it will update the back up with the changes you have made. That makes subsequent back ups much faster. Many of these apps also have scheduling capabilities: So set it up and it will do the back up automatically.
Example of such apps: Chronosync- but there are many others. Search on MacUpdateor the App Store
For more on iPhoto and file management see this User Tip:
https://discussions.apple.com/docs/DOC-6361
As you're using iPhoto 08 the Reveal command is on the Contextual Menu.