This is likely because the size of the image is too large to be uploaded, but the server was not providing a clear error message. The message that you get should now be fixed if this is the case. If you run into this, check the size of the image. If the width is greater than about 1600 (or is it 16,000) pixels, the system will automatically reject the image because it is too big. There's an easy fix for this though. Open the image in the image editor of your choice and scale down the size of the image, then save the file.