First you have to upload a pic to a site like Photobucket, Picasa, Flickr, etc. Then you need to get the pic's URL from the site. Paste the URL between [img] and [/img] without any spaces.
If you're on your computer, there's an icon at the top of the post window to Insert Image. It will give you a popup and you can paste the image's URL there.
Or you can still do it the same way as above also.
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