Above the "Reply" box, there is a button, about in the middle, labeled "Img"
That will put two labels in your post that say "img" and "/img" with each of them contained in brackets. You paste the url of the photo's location between those two (which is right where you cursor will be when they appear in your post).
Hope that made it clear enough.
If the photo is on your own computer, then you need to get it onto the web first. If it's on your own computer, you can put it on the site by going into the user control panel, hitting the "Gallery" tab, choosing "Manage personal albums," hitting the "create personal album" button, and then it will say "You don't have the permissions create your personal album" because they haven't activated that feature yet.
BUT, as soon as they do, you'll be able to establish a folder here and upload pics to it. Until then, you can't put a picture in your computer onto your post unless you have already established a photo site somewhere else on the web. Then you can use the "Img" button.