Upload your video to youtube first.
Download your video from youtube in mp4 format
INSTALL Opera web browser to your desktop
Use Opera to upload your downloaded video
Take snapshot, open snapshot and export it to 28KB size
Upload your 28KB size snapshot to thumbnail
Finish description
I used this method and it worked to upload this video https://steemit.com/indroduceyourself/@korneliusz/iytixfj8 (i couldn't of done this without the help of my dad 
