Have a look at the live demo at http://hasinhayder.github.io/ResponsiveGalleryWithFiltering. Resize the browser window and see how the grid items adapt to the new width. Select categories from the dropdown list in the top right corner to see how the filtering works.
Positioning the dropdown list was managed using media queries which you can see at the bottom of the <style> block.
One more thing to note here, a very important note perhaps. If you have used MixItUp before then you probably already know that every time mixitup refreshes the list, it actually removes the old list items and redraws them in the container. This phenomenon removes any events attached to the list items. To fix it up, you need to attach event listeners using $.delegate() method, as done in the example. However, lets have a look at that
var src = $(this).find("img").attr("src");