Let me step in here and toss out a shameless plug.
If the image is on the internet already:
Do what has been mentioned above. Right click the image, and copy the URL (The link) to that image.
Come here, and click the image icon (shown above in .Lee.'s post) and paste it into the box that will pop up.
If the picture is on your computer, you need to upload it to the internet first. I would suggest going to
http://www.actionupload.com , its fast and doesn't require you to register. Click "Browse" and find the picture on your hard-drive, select it, then click "Upload". The image will upload and you will be given a ***** code that you can copy and paste into your post on forums, and it will display your image.