To verify the condition posted would work ?

One question about enquire, I don't understand match:function ? Although I probably will stay with pure-Js.

enquire.register("screen and (max-width:45em)", {
if () { 
    match : function() {},      

match is if it meets the media query and unmatch if it doesn't. Pretty much the same as if/else

How did you find that you can add .src="img/small.jpg"; to a getElementById node ?

Ahh, I was expecting another link.

Ahh, I was expecting another link.

Yeah? Well... this was the link :)
All you need to do is replacing the image that you have as default in your HTML, right?

I like the new JsFiddle new look; it's clean. I can't debug my code ? I'm going to have to resort to here.

As far as I can see they removed the ability to show the result as a full page. is in that respect my preferable option.

document.getElementById("tleone").src = mqls[0].matches

You're targeting with 'tleone' the wrapper div and not the img tag, so this won't work.

I gave CodePen a try, I feel I have more screen estate with JsFiddle then Code Pen, there is less obstuctiveness with JsFiddle, the only hiccup is, what happened to error checking and the browser must refresh to remove the "post error".

I updated the code; CodePen notified me of a "unexpected token" before reverting back to JsFiddle, while jsFiddle, as mentioned, no error checking !