Trying adding:
img { display: block; }
to your CSS. Since an <img> is an inline element by default, its height is calculated differently as related to the default line-height value.
On inline elements, the line-height CSS property specifies the height that is used in the calculation of the line box height.
On block level elements, line-height specifies the minimal height of line boxes within the element.
Source: https://developer.mozilla.org/en/CSS/line-height