Here you are:
1. Click on "Choose File" button, in the lower right-hand corner, next to the green "UPLOAD" button
2. A file browser for files on your computer will pop up... choose the file you want and upload it
3. In the lower left-hand corner you will see a drop-down menu "Manage Current Attachments", with the number of attachments available in brackets... when the menu drops down, your file will be there
4. Next to your file name, there will be two icons... one with a green dot, the other with a red one... the green one, when hover your mouse over it will say: "Insert attachment into text editor", the other one: "Delete attachment"
5. Position your text pointer where you want the picture to be inserted in the text editor, then click on "Insert attachment into text editor"
There you go!
P.S. As a part of this exercise, I am inserting one of my pictures, hope you do not mind
[attachment=10097:Bass_Harbor.jpg]