To the right of the "post reply" button is an "upload a file" button, hit that and then a black box will pop up that says "choose file"
View attachment 3460873
Click "choose file" and find the pic you want. If for some reason nothing happens when you click "choose file" just close the black box and hit upload file again(sometimes it's lame and you gotta do that a couple of times)
After you choose your pic give it a minute or so and you'll end up with this
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Hit "full image" because everybody hates clicking on thumbnails.
Done, now you owe me $10