reported a violation

Discovered one of my double star images on a Canadian astronomy group's YouTube channel, used without my permission. In fact, it was used as the thumbnail image on five of their videos. 

I pointed out the copyright violation but offered that they could use it with credits and links. They removed the thumbnail from the videos in question. Later a follow-up private email was sent with apologies.

They said tried to use images on public domain sites like Wikimedia Commons or Public Domain Pictures, etc. but sometimes weren't sure of the source. Such a slipshod approach might get them into more trouble. 

In fact, they appear to have taken copyrighted infographics for their Jupiter-Saturn conjunction video from USA observatories and USA news agencies without credits or courtesy notices. The owners might not take lightly to that.

I understand Fair Use situations and use by a non-profit but I believe it still very appropriate and considerate for sources and credits to be shown.