I don't understand.
Google YouTube please think. Whenever someone uploaded a video, why did you allow them to "deviate the content" by adding whatever thumbnail they deemed fit? For example, I can upload a kids MTV song and add whatever thumbnail I wished by searching for the "best image" on my computer. What if I decided to add a graphic sex to the kids' song? How about adding a thumbnail with violent beheading in action? Can you see the irrationality? I don't really care if you had validated their accounts prior to allowing them to use the thumbnail function, or that you would promptly remove/inactivate their accounts if any violation had been detected. I only care for the safeguard that you will be implementing to ensure that thumbnail function is not being misused anymore. Why not allow thumbnail to be derived from the video being uploaded instead, and how about showing randomly selected thumbnail from the uploaded video (and minimising user intervention)? When a thumbnail for a video can be misleading, then the use of thumbnail is defeated, and thus, I don't see the point of using thumbnail, do you Google?
Now, as a parent yourself, can you see the risk of me keying search term, "Kid song" and end up with thumbnails in some results showing graphic sex, demented action scene, violent beheading snapshots, grisly ghost shots, and all in front of my toddler? Although the thumbnail is small in a 17 inch LED monitor, it ain't so when you are using YouTube with your Smart TV that might be 40 inch or more.
So Google, please do the right thing before YouTube ends up with more complaints than views.
Just a thought.