Removing border from all images.

This has been asked before and I have tried all the other suggestions but it hasn’t worked!

Can someone paste the code here that I need that will remove the grey shadow border from around my images please. I have managed to remove it from the custom header but I want to remove it from all images (inc the side bar).

I am using the Twenty Twelve child theme.

Thanks in adavance for your help.

Caryn x