Go to http://www.google.com/imghp at the right end of the search text box you will notice a camera icon.
Window your browser, open a folder with images and position both so you can drag an image from the folder to the browser window.
When you do that a Search by Image box will open drop the image in the box.
It will be uploaded
Alternately you can click the camera icon which will open a dialog box that has an upload image link.
Search will use the image to find images that match or are similar.