I think slob mentioned in another thread that forum photo posting was currently broken so you need to use a picture-hosting site. I use
www.postimages.org/signup.
Once you've uploaded your picture, you should be presented with these options...
Click on the blue 'copy' icon next to the 'hotlink for forums' address and paste it into your post.
If you preview the post before submitting it, the picture will appear in the preview if you've done it correctly.