Pictures are located under My Documents/EA Games/The Sims 2/Neighborhoods/N00X/Storytelling (X=Whatever number neighborhood it is - they go in order. ex. Pleasantview is N001)

When you post a reply, look below the "Submit Reply" section at Additional Options. There's a big button called "Manage Attachments". Press that and browse until you find your picture.

You probably want to right click in the Storytelling folder and have it arrange by Modified, then the newest pics will be at the bottom. Press View... then select Thumbnails so you can see the pics. Select the snapshot version because the thumbnail version is too small. Then upload it and post.