That's a known issue, best solution is to go directly to the image posting site used by the forum:
https://postimages.org/Upload it there, and then you can post the image link in your PM. If you use the
Insert Image button in the formatting options above the textbox (right below the "
B" bold button, first button from the left) and wrap the link with the img tag, it will show the image directly in the PM.