To add in images you must first add your images into your Images & Documents manager folder.
To do this please hover your mouse over the "My Account" tab and select "Images & Documents". A new window will pop up, please click on the "Upload" button (top left quadrant of the new window). From there you will be able to select your image and upload it into your image manager.
Then, when you are in your marketing piece, head to the "Add General Content" step, and please either click on the image icon in your WYSIWYG toolbar or choose "Insert > Insert Image". Your image manager will pop up, select your image and click insert.