function strpos (haystack, needle, offset) {
    // Finds position of first occurrence of a string within another  
    // 
    // version: 909.322
    // discuss at: http://phpjs.org/functions/strpos
    // +   original by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +   improved by: Onno Marsman    
    // +   bugfixed by: Daniel Esteban
    // *     example 1: strpos('Kevin van Zonneveld', 'e', 5);
    // *     returns 1: 14
    var i = (haystack+'').indexOf(needle, (offset ? offset : 0));
    return i === -1 ? false : i;
}

	function changeimage(elem){
		if(strpos(elem.style.background, 'white_arrow.png'))
		{
			elem.style.background='transparent url(_assets/images/white_arrow_down.png) no-repeat scroll 10px center';
		} else {
			elem.style.background='transparent url(_assets/images/white_arrow.png) no-repeat scroll 10px center';
		}
	}