No matter if you want to add your images as you are creating the post or after you have written all the content and add your pictures last. You need pictures for visualization.
First thing to do is make sure that your cursor is placed in your post in the area that you will want to put the image.
While you are in SiteContent you want to click on the media button. It is right beside the abc checker.
Once you have clicked on the image box, it will bring up a search bar that allows you to type in the the type of image you would like to use.
Below is a picture showing you how it works. I have searched "adding images" so that it fits this tutorial.
Once you have found a image you would like to use, you need to click on it. This will bring up another box where you will need to crop the image. You can click on the white boxes or the white lines to crop.
When cropping your picture you want to have an arrow and not something that looks like crosshairs or something to that effect.
If you try to crop with the crosshairs it will just move the cropping box around. Unless you only want part of the picture cropped then in that case you can just grab the box and put it on the part of the picture you want cropped.