To add in images you must first add your images into your Images & Documents manager folder.
To do this within your marketing piece, while on the add general content step, please place your cursor where you'd like to insert your image, or click on an image you'd like to replace. Then click the 'insert/edit image' icon from the toolbar.
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 select your image and click insert.