If you have two Macs, you may want to merge the contents of two iPhoto Library folders, one from each computer.
To merge iPhoto Library folders (I):
Network your Macs via AirPort, Ethernet, or even FireWire (using IP over FireWire, which you can enable in the Network preference pane).
On both Macs, turn on photo sharing in iPhoto's Preferences window (Figure 2.27). For more information, see "Network Photo Sharing" in Chapter 8, "Sharing Photos."
Figure 2.27. To turn on photo sharing, select Look for Shared Photos and Share My Photos in the Sharing pane of iPhoto's Preferences window.
Drag the desired photos or albums from the source Mac's shared album to the Library album of the destination Mac.
To merge iPhoto Library folders (II):
If one of your Macs has a writable optical drive (CD-R, CD-RW, or DVD-R), burn a disc containing all the photos you want to transfer from the source iPhoto Library folder. Then import them into the destination iPhoto Library folder as explained in "Importing from an iPhoto Disc," earlier in this chapter.
To merge iPhoto Library folders (III):
From the source iPhoto Library, export the desired images by album or film roll into folders (see "Exporting Files" in Chapter 8, "Sharing Photos").
Copy those folders to the Mac with the destination iPhoto Library folder via a network or FireWire Target Disk Mode.
Drag the folders into iPhoto to import their photos (see "Importing from Files," earlier in this chapter).
To merge iPhoto Library folders (IV):
Use Brian Webster's $19.95 iPhoto Library Manager, which claims to be able to merge iPhoto Library folders automatically, retaining all titles, keywords, ratings, and albums. Find it at: http://homepage.mac.com/bwebster/iphotolibrarymanager.html.
If you're burning to disc, use CD-RW media if you want to reuse that disc; use CD-R media if you want a backup.
When exporting via the third method, I recommend using the photo title as the filename or else you'll lose any work you put in titling your photos.