This is relatively easy.
Simply copy your entire iPhoto library to your external HD. Note that you are not going to just move them. If anything goes wrong during the copy, you still have the originals.
Once you have copied the library over to the external, hold down the option key and launch iPhoto. You will get a splash screen that asks what iPhoto library you want to use.
Choose "Other Library" and navigate to the external and the iPhoto Library. Choose it and that is all you have to do.
From now on, iPhoto will open using that Library.
Once you are satisfied that all your pictures are on the external, you can safely delete the other iPhoto Library.
Remember to add the external to your Time Machine backup routine, otherwise you could lose the lot if the external crashes.
Hope that helps
My G5 is completely full. Can I transfer my iphoto contents to an external HD? Using Time Machine won't help because it only backs up my photos so I can't add more to my iphoto library.
Or, is there a way to make iphoto an external HD application so the photos are only stored ion the external HD?
Thanks for any suggestions.