Common sense, they're used from all around the internet and individually tracking down the owner of every image you use in a video could be tricky.@RyanH what lets you assume that these other channels didn't obtain a proper permission in the first place?
I highly doubt they would contact every image owner that you use in your video. Networks don't really do anything like that for their partners. It's just being a little paranoid to think you'll get a strike for just using an image - don't worry about it. I've never really seen it happen, and I don't think YT would consider it appropriate to take down an entire video if the supposed owner flagged it.Well many people join networks that obtain general licenses for them ...