The design tool likely won’t let you add your design to the cart if your image you are adding to the design tool is larger than the design tool product canvas size. It is recommended that you utilize images no larger than 600 pixels wide by 400 pixels tall. You can crop your images for free here.
If you continue to experience issues please email marketing.mwp@morewithprint.com, so our IT department can investigate the issue you are having.