Once you have the link to your photo copied, in the Post Reply area will be a button above where you type that says "Img". Click it and paste the link in between the brackets that come up, the first one has img in it, the second has /img. Then when you submit it, the photo should show up in your post. You can click on the "Preview" button next to "Submit" to see what it will look like before you actually submit it.