When adding a photo directly to your post the easiest way is to click on the little mountain (yellow) picture scene in the Reply window. It is on the bottom line to the farthest right.

Click on that, accept scripted windows temporarily if your browser blocks it then click on the picture again.

A Script Prompt box will open up, insert the link in the blank box and click "ok".

After you do this, the picture should appear in the reply window. You can add text, etc, after this and click "ok" to post the reply.