It has never happened to me before. It seems like some press agency from UK want to sell my picture to newspapers in UK:
"I am hoping you would give your permission to use the picture with our story, you would be credited and paid if the picture is used in any of the papers we work with. We pay a straight 50% share of any money we can make selling the picture. This ranges from approx $50 if we get it on a couple of websites to $150 or so for each use in a newspaper. We work with all the main newspapers in the UK and look to sell each story to the main 9 publications. If you are happy we can use this exchange of emails as an informal contract, we don't need exclusive rights as you guessed. If you want to do the pictures exclusively with us and would like a contract you could upload the pictures through our website [edited] which has a contract built in"

I am not comfortable with informal contract but uploading thru their website requires granting them exclusive right which I cannot do for this picture.. I am reading their terms of use right now...

What do you mean, don't they define the license terms - even exclusivity as you've mentioned?

When I sell directly, I write restrictions in the invoice.  I know it isn't probably the best thing, but I don't have the knowledge of legal terms to make a real license document.