Zakeke automatically checks if the quality of the uploaded image is good enough based on the size of the entire print area on the product and DPI value that you require (by default, the DPI required is 300 but you can change it by creating a Printing Method to associate to the product). When the quality is not good, the customer is informed by a warning message that suggests uploading a better quality image. In order to allow lower resolution images, you need to set a lower DPI value by creating your own Printing Method.
This is how the message gets triggered:
The warning message will be shown if the image has a resolution (in pixels) lower than this threshold: covered area (cm) x DPI (set in the printing method) / 2.54.
You can prevent the message from being shown by disabling it in the section Settings > Messages and Alerts in your Zakeke back-office.
IMPORTANT: you can't change the way the quality check works for products sourced from integrated third-party catalogs as you can't change the required DPI for those products.
Read more here on how to manage messages and warnings within Zakeke.