To post photos:
When typing in the white box under the word "Reply", you'll see a gray bar with different icons. The icon that looks like a square with mountains is the button to upload photos. Click the button that says "Upload Files." Then give it some time to upload while you choose your size. Small is usually fine. Medium is good if you want to see more detail. I don't know why the Large option exists...it must make a REALLY BIG photo appear on the screen!
Hope that helps.