Document.Images() - HTML

this method (Document.images()) allows you to see the number of images you have in your webpage but I need my webpage to do somehing according to the number of images in another one. How can I use this method (or any other) to see the number of images in that external webpage?

I don't really know how document.xxx works so I tried few things. I added the other webpages url before or I changed "Document" with the URL,...but it doesn't work. Do you have any solutions? Thanks.

This is the part of my function were I'm having problems:

var x = document.images.length;
if (x == 3) //(if there are 3 images in the other webpage)
{
    //do somehting
}