Its pretty easy, first you will have to save the screenshots somewhere on your computer. Then you just have to add the screenshot(s) as an attachment to your post. Once your image has been attached, you are then able to insert them somewhere in your message.
But make sure that your image isn't too big, try to keep it below 200KB. (Not sure what the maximum size a image is allowed to be).
Also make sure you dont have any spaces in the name of your image as this normally causes problems. Its better to use "_" or "-" instead of spaces.
I hope this helps