It's easiest if you have something like a photobucket or flicker account.
I have a photobucket account so I'll use that as an example
Upload your pictures to your account, when you view the picture from the account you will see a button that lets you copy the address of the picture with the correct code prefix and suffix for it to be displayed in your forum post. Labelled 1 in the bottom left of this picture. Simply paste this into your post.
The other way is to use the image button on the forum post page. This will open a pop up box. It will look something like this, the button surrounded in pale blue is the image button.
You then copy the address of the picture (labelled 2 in the first picture). Select OK and it will insert the address with the correct code and it should look like this.
[IMG]www.AddressOfYourPicture.com[/IMG]
Good luck