If you need to convert the size of an image to be smaller, you can utilize an online resource to assist you in altering the size. For instance, you can use an application called Adobe Scan which is free and allows you to scan documents or convert existing images into PDFs, reducing the file size. The app works for iPhone, Android, and Windows.
However, if the size of your image fits the requirement and an error with the system exists, please contact support here so the technical team can provide a solution.