I have an article here that I believe will provide you with some information to resolve this issue:
Specifically, I think you just need to follow the step listed below:
Tip: If you need to assign the shared folder user or group account privileges to items contained in the shared folder, select "Apply to enclosed items" from the action pop-up menu. A confirmation sheet appears, click OK to complete the process.
It's also possible that this is a permissions issue within iPhoto. If this is the case, the issue can be resolved by following the steps in this article:
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