Make images responsive using css with WP reponsive theme

Ok, I'm using a responsive WP Theme and just discovered my images are not responsive. I understand why but my limited CSS is hindering me big time.

All the images have fixed width and heights. I want all images to be responsive so can I not simply override all img tags with new CSS to make them responsive? Do I need a widget to do this?

I've been trying simple CSS entries like

I can directly affect the size of each image using something similar to above but no matter what combination of width, height, max-height and %'s I use the images stay a fixed size and distort the page when mobile.

Would appreciate any help.

Thanks Damien