If you look at this screenshot you’ll see a box below the rectangle where you write your post. It’s labelled Attach Files. Tap on it and you’ll be given three choices. If your photos are in the Photos app, tap that selection and select the photo you want. Tap the Add button at the top of the page the photo will start downloading. Tap the insert button and the full button that appears below. Then you tap the Post Reply button to finish.