There are two common issues that can prevent your video feature image from uploading:
Firstly, you need to ensure that you have provided a 640 x 360 pixel image. All other image dimensions will fail to upload.
Secondly, your image file name be causing the issue. Try to rename the image file to something more basic i.e. "image.jpg" or "picture.png" and then try again. Some special characters in the file name may cause the video feature image upload service to fail - only alphanumeric characters are accepted.