The same as you would for a post to a thread.
See the FAQ here: http://www.wrongplanet.net/postp656262.html#656262
Which is to say - they don't come directly from your computer: you upload them first to a company like Flickr, ImageShack or Photobucket and include a link to the image in your post.
If you click on the 'quote' button as if you're going to reply, you'll see the '[img]' tags surrounding the test image I posted here.
Most companies (I use Photobucket) will provide the link ready formatted so you just copy/paste it into your post.

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Giraffe: a ruminant with a view.