Hello,
if you did not take the picture you may not use it.
Somebody worked to take this picture.
Either take your own photos or pay for it.
You cannot steal any content just because it is in the internet.
I have accidentally found my thumbnail on another YouTube channel. I have filed a copyright claim, the channel is now closed. I think he stole more than just my thumbnail.
He contacted me and said he found my image on google and thought it was okay to use it.
The thought that everything that I upload might get stolen sometimes takes the fun out of content creating for me.
I love making videos and thumbnails and pictures for my blog / social media.
I take the photos myself and I also buy stock photos to use in my edits.
I really hope that in the future there will be ways to track your videos and photos in the net to see if it has been stolen.
Thank you and have a nice day!