How to print $item separately, with different content, in views-view-grid?
Anyone still looking for this, that's how I did it:

First I've created a simple view with a block to show just the fields that I needed on top (let's call it view-top), striped all default elements and classes to make it clean and easier to work (not really necessary to clean it). In OP's case would be the miniature for the gallery.

Then I've made my main view with the fields that I wanted on bottom (let's call it view-bottom). In OP's case would be the full sized images. In the header of this view I've added a "Global: Visualization area" and called my view-top.

This way I had two views, one embedded in the other, and with a bit of Drupal's behaviors I've made the click logic.