First you need to host the photo somewhere like flickr or photobucket or any of many alternatives or on your own domain if you have one.
Upload your photo there.
Copy the complete url of the photo, paste it into your post,
Highlight the url and click the Img button (above the text box)
Preview your post to make sure it works and if so
The photo will appear as if by magic when anyone reads the thread
It's best to keep the photo fairly small, maybe no more than about 500px wide, otherwise the picture could take up a lot of screen width which tends to irritates anyone who has two windows open simultaneously.
It's also advisable to keep the resolution down to 72dpi (plenty for on screen viewing) so the photo is quick to load.
The sample pic above is 506px w x 639px h at 72dpi
To see it "as done" click the quote button at top right of this post.