I just went to Google Images and searched for a bunch of movies until I found a still that would be a little more obscure. Then I clicked on it, and it took me to the page hosting the picture. If you right click on the picture, you can select "properties", and it will tell you the URL address for the picture itself. Then just copy that link and post it as an image in the thread.
If that doesn't work, right click and save the picture to your computer, then upload to a file hosting site (I recommend Photobucket) and then link it from there. That's a lot more involved though.
(If that doesn't make any sense to you, just PM me what movie you want to do and I can do it for you. :) )
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