Sorting out multiple iPhoto libraries

Question

Hello
This is my first time posting on apple support....
I've looked through many discussions and can't find the full answer to my questions so am posting a new question (apologies, if I am duplicating something I missed). 
Ok, so we (Husband and I) are new Mac users so not very familiar with Macs in general, but so far liking the transition away from PC etc
We both set up iPhoto libraries under our own user accounts, naively thinking we could share each other libraries and be able to edit each other's photos.  We were hoping to be able to have access to all our photos, edit etc and thus avoid duplicates and generally be much more efficient in organizing our photos. 
I have subsequently discovered this is not possible as we had hoped, and from my research I believe the best approach is to have one library on a shared folder that we both can access and edit.  And we are aware that we can only have this one library open under one user at a time, i.e. one user needs to quit iPhoto before the other user logs in and opens iPhoto. 
So based on what I have read, my plan was to:
1) Set up a sparse disk image on the shared folder that we can both access with read/write permissions
2) Back-up existing libraries via time machine
3) Rename my iphoto library (let's call it W library)
4) Back up via time machine
5) Move W library to disk image
6) Move husband's iphoto library to disk image
7) Back up again
8) Use iPhoto library manager to merge both libraries into one "master" library
9) Back up
10) Use single master library from either account
However, this hasn't quite gone to plan and I would like to ask for some help. BTW, we have Mac OS x version 10.6.7 and iPhoto version 9.1.3.
Is this a sensible plan to achieve a shared iPhoto library that we can both edit (given that we have already set up libraries under each account that have different photos in)
If so, what is the advantage of setting up a disk image over just using the shared folder as is?
I got in a bit of a muddle with setting up the disk image in the first place, and by accident, didn't make it big enough for both libraries, so  I deleted it (I think) and set another one up.  I gave everyone read & write access via  "get info" in the finder.  Trouble is I didn't use very good names and now I'm not sure if a) I did actually delete the first one, and b) if I didn't actually delete it, I am using the correct disk image for the next steps.    Is there a way to verify which disk image is which, and delete the smaller one?
Regardless of this problem,  I ploughed ahead with steps 2, 3, 4, and 5.  I dragged my renamed library to the disk image icon I could see on the desktop. 
I then used the option key to choose the W library on disk image when I opened iPhoto in the dock and all I get is this message:  Caution The iPhoto library is on a locked volume. Reopen iPhoto when you have read/write access, or reopen iPhoto with the Option key held down to choose another library.
So, if you've managed to read this far,  please can anyone tell me where I've gone wrong, and how to sort this out?  Bear in mind I'm a new Mac user (although that's probably pretty obvious by now!)
Thanks in advance for any help

Answer 1

You don't need the Disk Image. Assuming you have copies of the Libraries you can delete them all.
For iPhoto 09 (version 8.0.2) and later:
What you mean by 'share'.
If you want the other user to be able to see the pics, but not add to, change or alter your library, then enable Sharing in your iPhoto (Preferences -> Sharing), leave iPhoto running and use Fast User Switching to open the other account. In that account, enable 'Look For Shared Libraries'. Your Library will appear in the other source pane.
Any user can drag a pic from the Shared Library to their own in the iPhoto Window.
Remember iPhoto must be running in both accounts for this to work.
If you want the other user to have the same access to the library as you: to be able to add, edit, organise, keyword etc.
Quit iPhoto in both accounts. Move the Library to the Users / Shared Folder
(You can also use an external HD set to ignore permissions, a Disk Image or even partition your Hard Disk.)
In each account in turn: Double click on the Library to open it. (You may be asked to repair the Library Permissions.) From that point on, this will be the default library location. Both accounts will have full access to the library, in fact, both accounts will 'own' it.
However, there is a catch with this system and it is a significant one. iPhoto is not a multi-user app., it does not have the code to negotiate two users simultaneously writing to the database, and trying will cause db corruption. So only one user at a time, and back up, back up back up.