Go to the picture you want to post and copy the whole address from the address bar.
Then come back here and click "New Thread" or "Reply", and in the window where you type you will notice a tool bar just above that has icons on it.
Click the square yellow icon that has a mountain and sun (looks like an envelope with a postage stamp to me). Then paste in the address to your picture in the little window that pops up. Click Ok.
Type your post, then click "submit reply".
If you want your text to show above the picture, type your text before clicking the yellow icon.