You need a picture hosting account, for example, Photobucket.
What you need to do is take your screenshot, then upload ut to Photobucket and save it in your account. Once done, and you either view an image or gallery, you should see some link options next to each image.
You need to select the image link address and copy the link (it should copy automatically but just in case it doesn't). Paste this direcetly in your post then preview it to make sure you've selected the correct link. From memory, it's usually the second link down and is called Image link.
Give that a blast.