To add images into a marketing piece, 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, 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.
On the initial popup, click the folder icon next to the Source field. A new window will pop up: please click on the "Upload" button (top left 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".
Back on the initial popup, the location of the image you selected will now be in the 'Source' field, and the native dimensions of the image will be populated. You can edit the dimensions to fit the image to the size needed in your marketing piece.