It's really quite simple.
Step 1. Host your image at Photobucket or any other photo hosting site.
Step 2. If you are using photobucket select the direct link under the thumbnail and copy it

If you aren't using Photobucket just make sure that the link you choose ends with .jpg, .gif, or .png
Don't use anything else on photobucket's link system because it usually doesn't work.
Step 3. Reply to the post you want to post your pictures into, OR start your new post and follow the instructions below:

(Click this photo to enlarge)

Last step. Click submit.