Rather than using a NSValueTransformer which will actually return immediately, subclass the collection view item. For each item fire the transaction in this subclasses asynchronusly. Keep a IBOutlet for the NSImage in your subclass so that when the response is received from the server jus set the Image using this IBOulet. With this approach when the item is created, the corresponding view controller comes into picture. Subclassing it gives you control over the view and how to load it.
I hope this helps.