Use the "Img" button above the text box. It will appear like this: . Put the URL for whatever image you want between the boxes like this: [img]URL[/img]. I usually preview my post first to make sure the image shows up.
If the image you want to post is on your PC, and not the internet, you'll need to upload it first. You can use a website like imageshack or photobucket to do that.
