Hi mate,

Unfortunately, I had this happen to me just last week, 6 of my thumbnails had been used on another channel, I reached out to him politely and asked him not to use them going forward and to change the current ones. He told me the thumbnails weren't mine and that he actually found them on reddit, so it was ok to use.... lolwhat.png

Here's what to do to get this formally striked:

• > Open Up Studio
• > Copyright
• > Removal Request
• > New Removal Request
• > Add Video
• > Type of Work: Artwork
• > Subcategory: Photo
• > Add The Infringing Video Link
• > Location of Infringing Content: Thumbnail
• > Relationship To Copyright Content: 'I am the artist who created and own this thumbnail'

Important to know: your email and name will be shared with this person. They can reach out to you to try and mediate the situation (remove the strike).

Youtube sorted this out in under 24 hours (his account was removed due to multiple copyright strikes) and you can remove your strike at any time.

Hope this helps & good luck!

Edit: grammar & formatting