A few things seem strange-
6) Logged in on first account (administrator)
7) Image already mounted??!!!
*??!!!
Is there anything else currently in the login items of the first account that could have mounted the image? Is there any other account (including a 3rd account) still active via fast user switching?
Also, normally when you look at the ownership of a volume that is marked "ignore ownership", whichever account is currently logged-in appears to be the owner. Your admin account seems to be named "Luca", but the only individual user I see on the mounted volume's permissions is "Nicoletta", even when you are logged into the "Luca" account.
For my own test, I used admin accounts "t1" and "t2". I first changed the permissions of the sparseimage file to read-write for all, and also checked the box for "ignore ownership" on the mounted image. Here are my GetInfo results for the sparseimage file, the mounted image, and the iPhoto-9 library, first as seen from account t1:
.
!http://i53.tinypic.com/2uj0f7n.jpg!
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and then as seen from account t2:
!http://i55.tinypic.com/258w8rb.jpg!
.
This way each individual user has read-write privileges to all three items.