Each topic has a main media frame which sits above the text in the final guide. In addition to this, you can add images within the text body using the image icon on the text entry box toolbar. Either select an existing image or upload a new one, then choose the size from the dropdown and click Save.
To change the size of an image displayed in the text box you must delete the current image using the regular delete key (just as if you were deleting some text) and then add the image again and choose a different size. Note that you won’t need to upload the image again, you can simply select it from your existing images.