"Upload attachment." Below that is a button that says "Browse." (If you don't see the blue bar or the Browse button, click the button that says "Full Editor.)
If you have a photo ready to go, use the Browse button to find it on your computer. Then click the "Add the File" button. If you haven't typed anything into the text section yet, do that. Once the file has uploaded, you get the little upload popup says to close it.
If you want the photo in the same box as your text, use the "place inline" button. It will copy something ugly that starts with "<a img src=" and ends with ">". Cut and paste that line wherever you want the picture to be, and hit Preview. If you're really brave you can hit Submit. Even if you post it "wrong," you should be able to delete your own mistakes and try again.
Looking forward to seeing your stuff.

Paul