When you post choose "Editor" mode and then you will see all the icons on the bar. You will see one that says "image" see screenshot - http://prntscr.com/iez62h
Then go to your picture location such as google images - right click on the picture you like and select "copy image location". See screenshot http://prntscr.com/iez8ie.
Then insert into the image drop down and paste and tick. then you are done. Hope this helps.