Just to confirm, do you see any error message when you try and upload one? Make sure your account is verified and see our recommended image size & resolution for custom thumbnails: https://goo.gl/T0AbeJ . Hope it helps!
Looks like you're in a small and brief experiment (.3% of viewers for a few weeks) so you're seeing auto-generated thumbnails instead of custom thumbs. This is part of an effort to test/improve auto-generated thumbnails for the many creators that use them:https://goo.gl/A7oDmr
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Note: 99.7% of viewers still see the creator's custom thumbnail and it shouldn’t affect a creator's channel in any significant way (watch time, views, etc). We just want to improve auto-generated thumbnails for the many creators that use them.
End of conversation
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