Posting is easy, Lisa. The most important part is to make sure your picture file (jpg) isn't too large. Resize it so that it doesn't exceed 700 x 700 pixels. (The file must be less than 200 Kb.) There are a number of ways to do that depending on what photo edititing software you have.
When you type out your new thread, or reply to a thread, scroll down the page to Additional Options. Click on the button that says 'Manage Attachments'. Use the Browse button to select the file to attach and then hit Upload. After that you can close that box.
You can preview the picture by clicking on it's filename before you actually submit the reply. Then hit submit. It's really quite easy.