I had heard something similar and so checked out the Flickr terms and conditions.
The Flickr terms and conditions directs to Yahoo T&C. in my case Yahoo Australia (so yours may vary).
There is a special sub-paragraph related to photo, graphics, audio and video.
This is my interpretation/summary of what it says ( I have no legal training/qualifications whatsoever). You do not grant them ownership. You do grant them the right to use the photo for the purpose it was made available. If you remove the photo you revoke their rights to use the photo.
It took me a couple of times to read. My eyes tend to glaze over when I start reading legalese.
I assume they need these rights. For example if someone retrieved one of your images in search, while they were in a public place, you might technically be able to sue Flickr/Yahoo for breach of copyright for reproducing your image in a public without permission, even though its just on someone's tablet in a public park or something. It probably lets them use your image on the front page of Flickr and stuff like that too. Maybe I'm naïve. I'm pretty sure they can't sell your image without your permission or anything like that.
If you are ever having trouble sleeping, google copyright law and start reading. You'll nod off in no time.