To add a photo by URL (like if you have the picture on a site like Flickr,) click on the photo icon. It's to the right of the bold, Italic, and underline.
Or you can go edit your first post, scroll almost all the way down and click on manage attachments. It'll be below the text box. You can either add links there, or upload directly from your computer or phone.
Whenever I add photos, it'll only let me upload one at a time.