Jeff,
Here is an easy tutorial for you.
First copy the complete url of the photo which you want to display.
Then click the button in the rely form that looks like a picture of a mountain.
A window pops up.
Right click on the highlighted http and select "paste' and the address is inserted.
Click OK and it is inserted into the body of your reply.
Note that I shortened the url in the last screen shot so it would fit in the window. Obviously don't do this as the photo will not show.
Steve