Hey, I had the exact same problem and here is the best solution (in my opinion). First decide which user will have the photos and put them all in there under your iphoto library. Next, open up finder and then open up the 'users' folder. Under 'users' open up the folder for the user that has the pictures (you need to be on this user's account when you do this). Now you should see all the folders for that user such as desktop, documents, downloads, etc. Find the pictures folder and right click on it. Select "get info". On the new window that opens up you will see "sharing & permissions" down at the bottom. All you have to do is select the plus button and add your wifes user name to the list. Once you have done that, she will be able to look at all of your pictures and even use your library for her iphoto app (so that you don't duplicate the pics and take up space). If you want to let her edit the photos (which I'm sure you better do!), then make sure under "privilege" you give her the ability to "read & write".
I know it is a lot to follow so don't hesitate to reply if you get stuck. I can try explaining it again. This will make it so that two or more users can access the same files such as pictures, music, documents, or whatever you decide to share. This saves you from having to use a separate hardrive just to share things like pictures. Hope this helps.
Enjoy
