This is a really nice theme but unfortunately it is not truly 100% responsive as the description claims. In my tests I uploaded images of various widths (some up to 1280 px wide) and then minimized my browser (Chrome & IE10) to see if the theme would adjust the image size on post pages to fit the browser window frame and sadly it did not. Images were simply cropped on the right hand side as I shrunk the browser frame. However, the theme is responsive on the home page and looks wonderful. This theme provides the closest style to a Tumblr blog that I've seen in WordPress.
There are several free WP themes that can do image resizing on the fly, which to me is the definition of "responsive". Some that come to mind are Frontier, Snaps and even the default twenty thirteen and twenty fourteen themes I believe.
So my next step is to try and figure out how the other themes are doing the image resizing effect and then see if I can add that code into a FluxiPress child theme.
Make no mistake, FluxiPress is truly beautiful and amazing, but with the image resizing problem it is not perfect. Fixing this will make this one of the best themes you can get for displaying images.