First open an account with photo bucket( seems to be the most common). And then use the download photos tab to place photos onto you photo bucket page.

Once the photos are on photo bucket, when you place the curser on the photo you wish to post you will notice a dropdown box with 4 small boxes. Click on the 3rd one down. It will say copied. Simply go over to the forum place the curser in the post where you want the photo and paste. When you preview the post the pic will show.

Just to add to what Chad said-the 3rd box down is the HTML code.