var imgDir = './images/';
var suffix = '_on';

function act(tagName,imgName) {
	if (document.images) {
		if (!imgName) { imgName = tagName; }
		var ext = getExt(document[tagName].src);
		var img = new Image();
		img.src = imgDir + imgName + suffix + ext;
		document[tagName].src = img.src;
	}
}

function inact(tagName,imgName) {
	if (document.images) {
		if (!imgName) { imgName = tagName; }
		var ext = getExt(document[tagName].src);
		var img = new Image();
		img.src = imgDir + imgName + ext;
		document[tagName].src = img.src;
	}
}

function getExt(src) {
	var file = src.substring( src.lastIndexOf('/',src.length)+1, src.length );
	return( file.substring( file.lastIndexOf('.'), file.length ) );
}