If you use a photo-hosting side like Photobucket or flickr, you can put multiple photos in a single post. If your photos are on your PC or phone, you are limited to one photo per post.

You should be able to add a second photo in a new post (or more than one photo in a second post, depending where your photos are stored) by replying to your original post, under Post a Follow-Up. [Sometimes GW's software thinks you're making a duplicate post, but you should be able to get around that by giving the new post a new title (in the box Subject of Posting).]