If your image does not upload during Step 3: Add Appstore Details of the app submission process, please check the dimensions and format of your image.
Ensure that you are uploading the correct image type to the designated submission slot. For instance, uploading an icon as an app screenshot will cause an error.
When uploading assets, the image extension must match the actual image type; otherwise, the upload will fail. For example, if an image is a JPEG but has a .png extension, the upload will not succeed.
Please read the Image asset guidelines for more information: