The supersized jQuery Plugin does magic image resizing with javascript in order to have full size background image, that fits. One thing you should remember is to not interfere with this resizing by your custom CSS rules.
One common CSS rule these days is max-width:100%; on all images - and this rule does disturbe the image resizing of supersized, with the result that your images look distorted.
To solve this add this to your supersized.css
#supersized img { max-width: none; }