I think it depends where the discussion is taking place. In a discussion here on the forum you can add a pic very easily, but in a privat discussion that's not so easy. Other then uploading it as privat pic, I don't know if there is another way.

You could always sent them the url and let them look it up themselves.

And if you mean you don't know how to post it here on a forum-discussion, then I refer you do what is stated in the FAQ under the button 'membersupport':

Question: How to insert image into forum or blog?

Answer: Go to the image page, look for button under the image that says "post it in blog or forum", hit it, copy the code that says "ubb for most of forums" and then paste it into your message that you are going to post into forum or blog.