You can either wrap image tags around the URL - if for example, you wanted to display a picture from the FF gallery, you'd use the URL for that photo with
wrapped around it. You could also use the 'insert image' command and enter the URL in the pop up field.
If you wanted to attach a photo to a post, e.g. directly from your hard drive, then you'd use the attachments icon (a paperclip). You then browse and upload your images