Thumbnails are fuzzy

[Gary High-Fruit]

I'm using paint.net. I've tried

1280x720px
and
1920x1080px

saved as jpeg or pdn.

The thumbs look sharp on a pic viewer, but after uploading, they are so fuzzy (at any screen size).

Any suggestions?

 

[McCoob]

It might be just youtube buffering images. Just guessing.

 

[Gary High-Fruit]

I forgot this update: I used a different PC, and the thumbs look just like my originals!
My PC - their still fuzzy, but other channel's thumbs are sharp. My pc is good tho: Core i5. An ASUS with an unusual screen dimension.

Just noticed! - the same thumbs on Twitter & G+ are sharp. But those thumbs are COMING from youtube. Strange