function mime_from_data(data) // Simple function that checks for JPG, GIF and PNG from image data. Otherwise returns false.
{
	if('GIF'==data.substr(0,3))return 'image/gif';
	else if('PNG'==data.substr(1,3))return 'image/png';
	else if('JFIF'==data.substr(6,4))return 'image/jpg';
	return false;
};
function data_string(data) // Generates the binary data string from character / multibyte data
{
	var data_string='';
	for(var i=0,il=data.length;i<il;i++)data_string+=String.fromCharCode(data[i].charCodeAt(0)&0xff);
	return data_string;
};

function load_image()
{
	GM_xmlhttpRequest({
		method: 'GET',
		url: 'http://www.domain.com/image.gif', // URL to image
		overrideMimeType: 'text/plain; charset=x-user-defined',
		onload: function(xhr)
		{
			var data = data_string(xhr.responseText);
			
			// We can now do what we like with the data, E.g.
			var base64_data = btoa(data); // Encode to base64 string
			
			// We can then turn the Base64 string into a image using Data URL's ( http://en.wikipedia.org/wiki/Data_URI_scheme )
			var data_url = 'data:' + mime_from_data(data) + ';base64,' + base64_data; // Make the data url
			
			var image = new Image();
			image.src = data_url;
			document.body.appendChild(image); // Put image to page
			
			// Or put the image to a cavas to manipulate
			var canvas = unsafeWindow.document.createElement('canvas'); // Canvas is created in unsafeWindow context to aviod security issues
			canvas.getContext('2d').drawImage(image,0,0); // Put image to canvas
			
			// Or we can send the image data to a server
			GM_xmlhttpRequest({
				method:'POST',
				headers:{'Content-type':'application/x-www-form-urlencoded'},
				url:'http://www.domain.com/process.php', // URL to script
				data:'image_data='+encodeURIComponent(base64_data),
				onload:function(xhr)
				{
					// You can then decode the base64 data and write it to a file using PHP etc.
					return;
				}
			});
			return;
		}
	});
	return;
};

window.addEventListener('load_image',load_image,false); // Make sure image has properly loaded