Have reviewed your install. Someone altered the images after they were uploaded and changed their filesizes. This causes the cached filesize to no longer match the filesize of the image. The result is the header returns the wrong content-length causing the page to never finish loading as it's expecting more data than it received.
As the cached values were intended for display purposes only I am implementing a fix for it to stop using cached values for header usage when outputting media. I've already installed the fixed build on your site where everything is now working fine. Bug ticket regarding this issue can be found below.
forge.joomlapolis.com/issues/6920