There are two ways:
First, if it is an image on the web, you can use the {img}url of the image/map{/img} tags (replacing the {} with []). However, many people dislike it if their bandwidth is leeched this way, so you might want to check if it is ok with the host.
The second is to attach the image from your computer. Below the reply box, you should see a box titled Additional Options. The second portion is titled Attach Files, which lists the valid file types, and the Manage Attachments button. If you push that one, a window will pop up with which you can browse your computer for the file you want to attach.
Note that the various file types have size limits. I don't know them exactly, but you'll get a warning message if the file is too big. If that happens, try to convert the file into another format (jpg usually works fine for the forums).
Edit: OK, seems your problem is taken care of.
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