Two ways. If you want to post a photo in your reply, it has to be hosted somewhere. I like Flickr. I wrote some instructions on using it HERE.
It will appear like this.
Or you can attach an image. It will appear as a small thumbnail in your post, which members must click to launch and see. To use that, select the USE FULL EDITOR option. That will open a new space for you to type your post. Down below all the smileys, you will see a paperclip that says "Attachment Manager" next to it. You can select that, and browse for and attach your photo that way. I have attached the same photo using that method, so you can see the thumbnail.
Both methods work...it's your choice.