Min/Max size of images with Responsive Themes

Hello WPMU Dev team, I am using a responsive Theme, and I am having some trouble with image sizes. Images when readjusted by the theme are sometimes too big, orr too small. What is the best way in HTML/CSS/Wordpress?etc. to put a min size and a max size for a images. That way my images will always be in the right band of size.

Thanks a lot!

Ronan