benji,
After hitting the two little rectangles to insert a photo into your post that Dunlin showed, over on right side the will see a button that says :Preview.
You can hit that and preview what your post will look like before you actually post it.
If you are trying to insert a photo that sits on an external web site (and not on your computer), you have to hit the little icon that looks like an mountain, and type in the url address of the location where your photo is sitting.
Steve Thomas