I got a request from a guy who'd like to use this image of mine for a print. I am normally open to non-commercial (simple attribution is okay) usage of my photos. So, I wrote "Drop me a flickr mail or contact at my email if you'd like to get the full image for a non-commercial print". However, I am concerned that some people might misuse this and use it for commercial purposes. So, How do I go about ensuring non-commercial use of a photo? Do I take any signed document from the person or simple email acknowledgement of non-commercial use will suffice?
If you say that the picture is for non-commercial use only then it's for non-commercial use only.
If you have a signed page stating this than the other person can't say he didn't know the picture is for non-commercial use only.
But, if that person is a jerk and uses the photo for commercial purposes anyway, what are you going to do? based on your comment you are from Bangladesh and the person is from Chile but is in Australia — can you afford a law firm that will handle this international lawsuit for you?
So, you can't "ensure" non-commercial use, but most people are somewhat honest and if you tell that person clearly that the picture is for non-commercial use and that this is important to you most likely everything will turn out okay (and it not then don't let it discourage you from doing this again with someone else, dishonest people do exist but they are a minority).
Just a little pointer from Europe.
In Europe a verbal contract has the same value as any written contract. I would assume this is similar in most of the world. The reason people prefer a signed contract is because these are difficult to fight and you don't end up in a "I said" or "I didn't say" debate.
Further, I've seen the suggestion to send an offending party a bill several times, on the idea that you just charge them a fee - or alternatively they have to stop using the image etc. Now how viable that is in your case I really don't know. Personally, I would say that the moment you have any form of written agreement, you are in a legally binding contract - and if that says non-comemrcial use it says non-commercial use - period. Now assuming your "customer" would break the contract, you can send a lawyer after him, or report it to the police, etc. but the question that may be asked is what economic damage was done to you. If you never sell any images and someone sells a print at a high price, it may be hard to prove that you have financially lost out for example - as a result image infringements also often won't go beyond a "cease and desist" order from a court, simply because it isn't worth fighting over it. There are exceptions to this - primarily press and tabloid related, but these relate to images that sell for several thousands a piece. I suspect if you wanted to have an extra deterrent you could possibly include any "damages" in the contract, however you should consult a lawyer over this because I'm not sure if you can.
So what can I conclude - it is really a matter of trust and people respecting contracts. Nothing else.
Finally, if you are willing to pay a step yearly fee, you can possibly try this against web use: http://www.digimarc.com/digimarc-for-images (Note: I don't, photography is expensive enough for me already, but then I normally don't print anything nor do I get requests for copies of my images.)
Edit: It looks like their price came down a bit, I'm still not sure whether I'd be willing to pay it though..
I would have a simple agreement that is electronically signed stating the permission and use for your photo. That way both parties are covered and there is no later issue of "he said - she said." It was clearly stated in writing.
A service like this is available for electronic signing.
Here are a couple sites that may help you along with the agreement. I do not know these sites or how accurate their content so use with caution.
Also with different countries coming into play that might make things a bit sticker but it still should work okay as most countries adhere to contract/copyright law.
What you said shouldn't be able to be interpreted in even the most loose way as granting an unlimited licence for commercial use by simply sending you an email. If the other party wanted to take it to the wall and get lawyers involved then the issue will not be about who said what but who about who can afford to fight the longest.
However the only way to enforce non commercial use is to get a formal contract stating the exact conditions of use signed by both parties. This should apply to US and UK law.
P.S. When asking or answering a legal question you should specify which country it applies to.