1. Go to your PM box
and open the Private Message you wish to post.
2. Press the keys "Apple + Shift + 3".
3. Your Mac will automatically save your screenshot on your desktop "Picture 1.png".
4. Go to https://imgur.com/
and press the "New Post" button, then "Browse" to select and upload your image.
5. After a short time, you will end up with a page like this. Click the little arrow, then "Get share links", then click the "Copy" button under "BBCode (Forums)".
6. Go to the topic or Private Message you wish to post the image in, and in the message or post body, paste the link (hold command and press V).