The first thing you need to do is upload your image to an image sharing website like Imgur. Next you will need to find either the direct link (ending in ".jpg" or ".png") or the "IMG" link (pictured below)
If you have a direct link, you will need to surround it in img tags. You can do this by clicking IMG icon in the top right corner of the post editor.
Your link should look like this:
if you have the "IMG" link, you can paste it directly into the post editor.