Switching off Images for Mobile View

In all views, this fluid grid site shows three images:

http://gmjones.org/dw/index.html

I assign the class 'class="hide_mobile" for, say, '_images/calculating.jpg'. But the image dissappears in all views.

Tried this in Dreamweaver and outside of Dreamweaver via Notepad++.

The top banner has been sussessfully switched off for mobile view. The only difference I can see is that it was a DIV that had been switched off and not a specific element, as in this case.

So, I have failed to acheive this effect in both Dreamweaver and Notepadd++!

Any ideas?

Files:

http://gmjones.org/dw/files.zip