unable to have my custom header image render at higher than 1600×900

Hi, I have a website built with euphony pro at www.mattimac.com -- and when I first set up the site, I was able to have a 4k image (4096 x 2304) in the header, although the recommended size is 1080x1920. I recently had to make some changes to the header file, then I re-uploaded it. Unfortunately now, although the original file (at http://mattimac.com/wp-content/uploads/new_-1.jpg ) is on the server in its original size, the browser keeps rendering it at 1600x900, whatever I do, with a file size of around 239kb. It will not render it any higher, although Wordpress itself generates higher-quality images. Even if I copy-paste the image address directly into my browser, so it isn't set into any div or wrapper code, it will not show at the original size or even close to it. I've tried emptying my browser cache for the website and adjusting .css and .php values and Wordpress maximum sizes, but nothing helps. I'd like the image to render at its original size, or very close to it. 239KB and 1600x900 is tiny. A big thank you in advance, Matti