First you need to see if the IMG code is on, not all forums allow image posting. If the image is found on your computer's HD then you can use the attachment option that you will find when you open a reply box. If the image is hosted outside of your computer on another site then you can right click and copy the properties then use the insert image button found on the toolbar (looks like a yellow envelope with a mountain on it). Click on that, erase what appears and then paste in your properties and post.
You can post an attachment here, but cannot insert an image. Try the Test Board to practice.