function resizeImgOO(el)
{
	function imgRatio()
	{
		return (el.height / el.width);
	}
	function holderRatio()
	{
		return (el.offsetParent.offsetHeight / el.offsetParent.offsetWidth);
	}
	function fitToContainer()
	{
		if(imgRatio>holderRatio)
		{
			el.height = el.offsetParent.offsetHeight;
		}
		else
		{
			el.width = el.offsetParent.offsetWidth;
		}
	}

	this.imgRatio = imgRatio;
	this.holderRatio = holderRatio;
	this.resize = fitToContainer;
}
var img = new resizeImgOO(document.getElementById('yourImgId'));
img.resize();