Do you want to add the photos to your posts or to your gallery? There are some announcements and stickies in the Site help forum and the Show Off forum about adding photos. Also, the photos have to be 100 KB or less. If the pictures are bigger than this, do you have Photoshop or another program to reduce the size? If the picture is too big, I think you'll get an error message. You can also click on the blue sentence under Add an Attachment, "Allowed Extensions and Sizes".
If the photo is already on your computer and it's 100 KB or less, when you are in the reply screen, scroll down to the Add an Attachment block. Click "Browse" across from Filename, and it should show you all the documents in your computer. Click on the one you want to include in your post, and you'll come back to the reply screen, with the name of the picture and its pathway in the Filename box. This pathway is not shown in the post. Then click "File Attachemnt" (add a comment if you want first) and click Submit. This site doesn't show previews of pictures for some reason; I think Fishfreek and Reefrunner said it's a bug that they have to fix, maybe the next time they do some site maintenance. So for now, you won't see a picture preview, but it should be there when you submit.