UPDATE (9pm PST): The inability to upload a developer image has been resolved. You should be able to upload and image and progress to the next steps of the submission process in the developer console. Thank you again for letting us know and your patience while we fixed it.
(4pm PST) We have been informed of an issue in the app submission process in the developer console affecting the ability to upload the requested developer image. We are aware of the problem and working on a solution. We will update the post when it is resolved. Thank you for your patience.