hunnsdon on "[Theme: Twenty Eleven] Responsive Images"

How do I get images to behave responsively with Twenty Eleven (Child Theme)?

I have placed a background image on every page. I have tried several ideas to affect size when using smaller screens. including using percentage sizes instead of pixels.

I used the css to style this individual image.

img[src="url to image"] {
	border: none;
	padding-top: 0;
	margin-top: 0;
}

I have tried placing css in here:

@media handheld,only screen and (max-width: 767px) {
	img[src="url to image"] {
	width: 23.55%;
	height: auto;
}

..with no effect! My browser size is 1024x768. where am I going wrong?