Easy to do. You will have to upload the picture to a website such as Photobucket first.
When you have a URL for the image (eg.
https://i.servimg.com/u/f85/14/05/16/33/mainlo11.jpg) click on the image icon in the menu bar (above the box you write a message in) or press 'alt+p'
It will open a box which you paste the URL into.
Press OK and it should hopefully work!