// for main menu
function swapMenuImage(event) {
	var el = event.target;
	if (el == undefined) el = event.srcElement;
	if (el.tagName == 'IMG' && el.id != selectedMenuId)
		swapImage(el);
}

// for all images
function swapImageManager(event) {
	var el = event.target;
	if (el == undefined) el = event.srcElement;
	if (el.tagName == 'IMG') swapImage(el);
}

// general
function swapImage(imgEl) {

	// get img filename
	var path = getRelativePath(imgEl.src, 'images');
	var file = getFilename(path);
	
	// split file in filename + extension
	var fileParts = file.split('.');
	file = fileParts[0];
	var ext = fileParts[1];

	// current button state
	var isActive = false;

	// drop 'on'-suffix, if any
	fileParts = file.split('-');
	var last = fileParts.length - 1;
	if (fileParts[last] == 'on') {
		file = '';
		for (var i = 0; i < last; i ++) {
			file += fileParts[i];
			if (i + 1 < last) file += '-';
		}
		isActive = true;
	}
	
	// determine new path
	if (isActive) file = file + '.' + ext; // set to default
	else file = file + '-on.' + ext; // set to active
	path = getPath(path) + file;

	// set new img src
	imgEl.src = path;
}