detect if images are allowed in browser

image detection.js

/**
Without waiting in browsers Opera and Webkit:
	if pics are allowed
		'img.complete' puts on 'true' only after event
		'onload', 'onerror' or 'onabort'
	if pics are not allowed
		instantly 'img.complete' becomes 'true', and 'onload' doesn't occurs
у IE:
	if pics are allowed
		property 'readyState' [*] changes two times
		and trows event 'onreadyState'
	if pics are not allowed
		property 'readyState' has value 'uninitialized'
Firefox:
	if pics are allowed
		instantly 'img.complete' becomes 'true', then throws event 'onload'
	if pics are not allowed
		also instantly 'img.complete' becomes 'true',
		after that throws 'onerror'
		
Таймаут приходится выдерживать, во-первых, для того, чтобы Firefox успел
 сообразить, какое событие ему выбрасывать, во-вторых, для того, чтобы IE
 успели проинициализировать свойство readyState.
		
[*] In IE all tag objects with loadable data have property 'readyState'.
    It included in HTML5 specification. (Therefore, will work in future
    browsers, but not fact, that in the same way as in IE.)
**/
(function() {
    function init(){
        var img = new Image();
        var imgAllowed = false; // variable for detect if event occured

        img.src = "http://www.software-dungeon.co.uk/icons/109012_bug.gif";
        // wittingly downloadable pic (for preventing onerror event),
        // better from own server
        img.onload = function() {
            imgAllowed = true;
			// event occured,
            //therefore, pics are allowed (works in all browsers)
            alert("Images allowed.");
        };
        
        function checkLoading() {
		 //pic completed, but event hasn't occur (W3C), or it's status
		 // 'uninitialized' (IE), therefore, pics aren't allowed
            if((img.complete && (imgAllowed === false)) ||
                    img.readyState === 'uninitialized') {
                alert ("Images aren't allowed.");
            }
        }
        
        // check with timeout (weak place in code)
        window.setTimeout(checkLoading,200);
        document.body.appendChild(img); // for beauty. Opera calls event
        // onload for this instruction, even pics are disabled.
    }

    if (window.addEventListener) {
        window.addEventListener("load", init, false);
    } else if (window.attachEvent) {
        window.attachEvent("onload", init);
    } else {
        window.onload = init;
    }

})();