To add an image to your post, tap the tiny icon on the right just over your keypad. It looks like a small photo. Choose an image or add an image to the media library there by tapping the photo icons on the bottom left. You can also add take a new image by tapping the camera icon.
If you want to select more than one image, tap the images and press on them for a second or so (called a longpress) until a number appears. Press it again to "unchoose" it.