Hi,
I just had the same issue when using WP Supersized with the Responsive theme.
The problem occured because the theme was setting a max-width:100% to ALL images. Since Supersized is outside of the container div I just simply put that identifier before the img tag, as such:
#container img {
max-width: 100%;
}
Which makes the max-width apply to all images in the container, but leaves the Supersized images alone.