Not to be confused with Google image search, this option allows you to use an image as the search term.
Visit
https://images.google.com/?gws_rd=ssl and you can drag and drop an image into the browser (only works on Firefox and Chrome browsers).
It then tries to match the image to the website that it's on, plus find similar images. So if you had an image that you couldn't remember where you got it from, drop it into that webpage and hopefully it can trace it back.
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How to post pics: When writing a message, scroll down and use the "attach file" option. Where possible, a photo sharing service can be used such as Flickr, Imgur etc. The image location (usually ending in .jpg) can be copy and pasted between the ][ brackets that appear when you click the 'Img' button (you can find this button at the top of the message box).
How to post youtube vids: Click on the 'youtubeHD' button at the top of the message box and in the middle of the ][ brackets that just appeared, copy and paste everything from the youtube web address that appears after the = sign i.e if this is the url:
https://www.youtube.com/watch?v=K2cNqaPSHv0 just copy "K2cNqaPSHv0").