Hi, not sure what the scope of your support is (I'm new to WPUM!) and this may not be in your remit but...
On my site home page... http://www.lifeessentials.org.uk/ – the main background image to the hero is pretty lo-res. The original selected image is 2000px wide but when I check the console it's loading in a tiny version (http://www.lifeessentials.org.uk/wp-content/uploads/2017/02/night-sky-wide-col-2000-uai-516x267.jpg).
I do have WP Smush installed and I 'think' this wasn't doing this before that (though I was having some issues with WP Fastest Cache (now deactivated). Can you think of anything that would be causing my install to show a smaller version than I want? Is it a WP Smush setting or something else?!