since I keep getting asked by different people if I can just send them the digital file to a photo of mine they've seen in the news, I'd like to ask:
How do I explain politely that I'm trying to make a living of this, and will only sell narrow rights to an image in proportion to an agreed upon payment?
From experience I'd advise that you should not explain anything (politely or otherwise) when it comes to people wanting you to work for free.
Explaining things just gets you into a situation where people (usually people who want everything for free) take it as an invitation to challenge your position regarding payment and licencing which will waste your time in a way that you cannot charge for (easily).
Instead, just create a standard (but polite) 'thanks for your interest in my work' type message and a form to request the image, usage terms and preferred payment method that they'd like.
You have, as a professional, received a request from a potential customer.
The response from you should be a written quotation stating your price and other commercial terms.
Simply telling them isn't rude. Most people would only be asking because they didn't realize it was a for-pay product in the first place. So just let them know.
Them: "Can you send me a link to the digital version of that photo?"
You: "Actually, selling those photographs is how I make my living. So I can't just give the source file to you, but if you want to talk about purchasing rights/licensing/a copy then just let me know."
If you store your images on an online repository, then just say “Sure!” and then just provide them with a link to the repository where you store them. But ensure that it is one where they will have to pay to download or pay to view anything larger than a large size thumbnail!
It will be very bold of someone to come back to you and ask for it to be given to him or her for free!
I would suggest asking why the people want the file, and what they intend to do with it, and then base your response upon what they say. If the person would be happy with having a low-resolution or significantly-watermarked version of the file containing a notice "Full-resolution version available for license at awesomephotos.example.com", handing out such a file might provide sales leads which one wouldn't get otherwise.
If the volume of requests one receives gets excessive, one could formulate a policy, post it on one's web site (e.g. "The photos on the free-sharing page may be freely distributed in unmodified form; watermarks and captions must be left intact except when rendered as thumbnails. Thumbnails of XX size or larger must include legible copies of the full notices; those between size XX and YY must legibly incorporate [some shorter notice]. Those smaller than YY are under no obligation. Please do not ask for free versions of images not posted here; if an image does not appear here, please assume that I am either not interested in making it available, or am already working on doing so.") and then respond to any request contrary to that policy with a form letter identifying the web page and repeating the policy for the benefit of anyone without a web browser immediately available.
"Thank you for your interest in my image! Please provide specific information regarding your intended usage of the image and I would be delighted to discuss licensing terms with you. Thanks!"
