The newer IE versions indeed resize the photo automatically to fit your screen, but they only do this if the window ONLY holds an image. If you've got a page with an image on it then the HTML code in the page tells the browser how big the image should be. If nothing is specified it is shown at full size, this is the case when you insert an image in a posting on a forum.
So indeed the best thing is to resize the image first, and then insert the smaller image. It's also kinder on those souls out there still using older IE versions or different browsers, as these may not have the handy function described above.