Hidding elements on mobile phone and tablet in portrait position

Hello,
In my website there are certain items (photo’s etc.) that are not suitable for display on a mobile phone (or tablet in portrait position) and I would like to hide these elements.
I have tried to add an extra class to the Responsive Design section of style.css, as shown below:

@media all and (max-width: 240px) {
	.verberg {
		Display: none;
}
}

However, when I add this class to the <div> that I want to hide, it does not work: it is still being shown. Can you help me and indicate what I should do to hide a particular <div> when the site is being displayed on a mobile phone?