Did you try to restrict the file chooser to .jpg ?
(Via the "Show open dialog" and then adding ".jpg" to the restricted file extensions)
I don't know any more solutions, you would probably have to ask a developer.
It might be also possible (as I already mentioned) that you simply can't not show the other options since your app is (though it is wrapped in NW.js) still coded for browsers and is, therefore, restricted to browser's controls.
Those may not include the possibility to open the gallery directly.