Ah, well I can explain that!
Find any random picture on any random internet page, and right click. Go to save image location. Then come back here, you may as well practise in this thread, and go [img ]"image location"[/img ]. So you need to not put in the spaces in the [img ], that's just so that I can show you the code, and for "image location" press CTRL and V, this will paste in the image location you want. Preview your post and the picture should be there.
You can see how I've done it above, too. Go that that post where I showed you the padlocks, and click on the quote button. When the next screen comes up, click on the button on the right hand side of the edit box that has the A/A on it, a plain A over a fancy A. Click on that to go from rich text to html and you'll see the BBcode which is the code we use on forums, the stuff with the [..]'s.
Another way is to copy the image location as described above, then come back here and click that yellow square above the reply text box that has mountains and a sun. That's an image inserter. When you click on that a box comes up, press CTRL and V to paste in the location, click OK and hey presto! It's quicker to do it yourself though, but if you get stuck that's there for you too.