The best and easiest way to add an image to your post is to use the file attachment feature. When you use this option, your image will be put into your post, and it will go into your personal library for easy access elsewhere on the forum (max file size is 250mb):
Alternatively, you can upload to a hosting site, like https://imgur.com/ and then simply copy the image (copy/paste) or copy the image URL and paste it into your post:
Note: You can do this with any image from any website, but be VERY careful about this, as if the site owner changes the image, it will change in your post as well! Best practice is to upload your own image (like to imgur).