Hi, I use picture libraries (for news content) every day so may be able to offer some relevant experience.
TBH, it’s an increasingly grey area with the growth of user-generated content and social media such as twitter/instagram.
If your picture is on Twitter, for example, some agencies will put it on their site and charge media for it – with a disclaimer that they don’t own the rights.
So you may be better off selling the pic to one agency, who could then defend the copyright.
Unfortunately I don’t have experience on the photographers’ side to advise of rates etc. But you are welcome to email me to discuss agencies’ reputations if you’d like.