Bentneedle, to post a picture, you’ll need to upload the image to a third party hosting service, like imgur, photobucket etc. Once uploaded, you can copy the link to the image and then past that between some image tags. Add the image tags to your post using the post tools button on the far left that looks like the Mona Lisa.
Alternatively, you could also post a link to another website where the photo is, Facebook page, a blog or whatever.