You know how to take a screenshot yes? If you don't, go onto whatever you want to take a picture of, then press 'print screen' on your keyboard, it's just above the 'Insert' button.
Save it to your computer with Paint, then edit it if you need to. The most basic way of doing it is uploading with an imaging site, such as TinyPic.
Just upload your screenshot, click on it after it has uploaded, then get the link for it. Go back to your thread you want to put the image in and look for
when you click it, insert your link and then press ok. Your image should appear.