jquery.flexslider plugin, which works fine.
But for screens < 600px I do not want to load the plugin. I tried several scripts and ended with the following, which I thought works best.
I inserted it after the CSS and the JS tags in the header, but unfortunately it works only on second load, not on the first neither with F5 (tested in IE 11 and in Chrome 45.0.2454.101).
Because of cross-browser differences, you cannot always rely upon document.head already being there. Use document.getElementsById instead to get a reference to the element.
Also, I noticed that you were not using http or https on the front of the URL.
After testing this out, it does work in IE and in Chrome so long as I use a valid URL. As a last step, make sure that your URL is valid by pasting it into a browser.