General feedback

[unclecheese]

High level stuff

• Writing directly to stores is an anti-pattern. I might consider something like galleryActions.initialise() in the gallery constructor.
• Consider a view/container design pattern to decouple the application logic (actions, state from stores) from the UI. The container has no props, but has store state that it applies to the single "view" component it is responsible for rendering. It listens to events from the view component and calls actions. This would liberate you to take something like the item component and move it to a completely different area of the CMS, used in a different context.
• Probably should start thinking about some shouldComponentUpdate hooks. Although React diffs the VirtualDOM and makes minimal DOM mutations, it still takes a lot a work to calc that diff, and shouldComponentUpdate can bypass that all together, if, for instance, newProps === prevProps && newState === prevState.
• One way that check can be easily defeated is through function binding in the render() method, e.g. <button onClick={this.handleClick.bind(this)}>. Every time you bind, you create a new reference to that function and therefore prevProps.onClick !== newProps.onClick every single time, defeating your shouldComponentUpdate.
• Consider moving XHRs to the action layer. Things tend to scale better that way, as it becomes increasingly more likely that multiple stores will want the result of a server call.
• Consider creating an API wrapper class singleton to tidy up all the jQuery.ajax.
• Consider creating two action types: server and view, to manage optimistic updates and fetched data:

createFolder: (data) {
    galleryDispatcher.dispatch({
        action: CONSTANTS.ITEM_STORE.CREATE,
        type: CONSTANTS.VIEW_ACTION,
        data: { title: 'New folder' },
        silent: silent
    });
    API.post('/folder/', data)
       .then(response => {
            galleryDispatcher.dispatch({
                action: CONSTANTS.ITEM_STORE.CREATE,
                type: CONSTANTS.SERVER_ACTION,
                data: response.body,
                silent: silent
            });
        });        
}

This definitely starts to get verbose, and I would recommend creating some factories to generate these actions.

Minor bits

• Consider removing var keywords in favour of const or let to provide some control over mutations
• Consider using Babel stage 0 to allow class properties. This provides two very nice bits of syntactical sugar:

static propTypes = { 

}

over MyClass.PropTypes = {}

and

myEventHandler = (e) => {

}

over this.myEventHandler = this.myEventHandler.bind(this);

[flashbackzoo]

Awesome - thanks for reviewing this!