1. To insert an image, scroll to the very bottom, where you see "Image" button. Click on it.

2. You should see something like this. To insert an image select it and press insert in the top right corner.
You can also upload an image in the bottom by pressing "Browse". Once you do that, you can select an image from your computer. After you have selected the image, press "Start Upload". After the image is uploaded, you should be able to find it.