Targeting Images for Responsive Desgin

Hello,
I am trying to get certain images on my site to become responsive but am having great difficulty finding anything to help me. What I am trying to do is target images I have inserted into pages on the site to make them shrink along with the page as the page adapts to different devices. Right now they stay the same size and just topple over one another then stack. I already tried the Simple Responsive Images plugin but can’t get that to work with what I want. Anyone know a way I can get this done?
Thanks in advance!