To post a photo on the Forum, the image needs to be hosted somewhere on the web (ie not just stored on your computer). You can get a free photo hosting account at PhotoBucket.com

All you then need to do is right click on the image and then click "properties"
A window will open up giving you details on the image, including it's URL (starting with http://) Just copy that URL and then, when you are in posting mode here, you will see a series of little buttons above the box that you are typing your message into click the one that says Img then paste your image URL and then click the Img button again

that should leave you with the URL pasted in between a [ Img ] and a [ /Img ] tag post away and your image will show!

PS it is a good idea to post images that aren't larger than around 400x400 pixels so the thread doesnt get distorted