A picture has to be uploaded onto the net before it can be viewed by other people. I'm not sure on this board if you can PM with a picture that isn't uploaded, I'm not sure if the PM have an uploaded program that uploads to the site.
The Private Message function does not permit attachments.
You can attach an image to a public post, send the PM with the link to the attachment, and then edit the post to remove the attachment after the PM is read and the recipient download the attachment.
But it's usually easier and more secure to exchange e-mail or IM addresses and exchange pictures directly.