What operating system?
I don't think there is a way to just copy and paste it. Basically, you need to get the picture saved as a graphics file, and then you will need to upload it to the forum using the "Manage Attachments" button that appears below where you write your post. Then you need to copy the link of the resulting attachment, click on the little picture icon above the place where you write your post, and insert the link in the box that pops up.
Alternatively, if you can get the picture posted online somewhere, then you can use that link with the little picture icon as described above.