When you are in “Visual” Editor you will see the following icons.¬† Click on the icon to Insert Image.¬†
This will bring up a small blank box.¬† Wait for a moment and you will see this.¬†
You can now choose your image from an URL or click on the camera (blue box) to upload from your computer.¬†
You can add a description and dimensions or leave it blank.¬†
If the picture is too big you can later edit it by clicking on the pencil and changing one of the dimensions.¬† Just make sure to tick “Constrain proportions”.