You're best off using a photo-hosting service. The best one is here:

http://prntscr.com/

Click on "download" on the bottom. Then, all you have to do is hit your "Print Screen" button on your keyboard, and size the image, and upload it to their site.

when you get the link, Open it, then right-click and Copy the image URL. Then you can paste that image url between [img] and [/img] tags as usual...

You can practice in the Help forum...