Firstly please don't stop posting. I have a feint belief that your inspirational adventures will one day inspire some spotty teenager who is addicted to the t'internet to realise there is a world of adventure outside his house for little more than the cost of his XBox and games. A real
adventure, where you get cold wet and genuinely invigorated rather than one where you get carpal tunnel and astigmatisms.
As poly says it should be possible to search for image 'borrowing' and if there isn't a service that exists to do it I'm sure something can be cobbled together with some of the geekery found on the t'internet... Universities certainly already do it for text plagarism. However, as whisper suggests a watermark or signature on a photo makes a difference. Personally I prefer a signature as I think its a classy way of saying "I think this photo is good enough I want people to know I took it" while more subtly saying "steal my photo, you'll be crediting me". I'm sure a few of us have borrowed a photo from the web for a presentation etc over the years. The ones that are signed or have a water mark are less likely to be borrowed in my experience unless you want to say "this isn't my work".
A text footer in your posts that says something to the effect of "Hope you like what I have to say, but if you want to quote it else where please ask first. If its not for profit I will usually agree." You may also want to look at some of the Creative Commons Licences (https://creativecommons.org/licenses/
) which help to avoid mis-interpretation of what you are happy to allow. It sounds like you might want to say your images and words are CC-BY-NC-ND and that you may consider other uses on request?
On your email to the author. It doesn't hit the right tones for me. I'd be far more tempted to look for a template take down or licence fee request letter which I'd send with the expectation of reaching a happy compromise where they publicly apologise on their website for using your material, credit it correctly on any use, and if they are making any money from it pay you a typical licencing fee - that might not be much you'd need to decide what proportion of their income was because of your photograph... If they wanted to negotiate on price they should have done it before using it. If you put the images on a stock photo site you probably get 20-30p per use! But if there are 10 photos thats £3.00. The more photos they've used the more they need you ;-).
I doubt the law has changed. I very much doubt the law can recognise "Social Media" versus "The Rest of the Web". Is this site Social Media? BUT what I do suspect is that you may either explicitly or through an implication of a statement in the site T&Cs that may say "No content you upload shall be copyright" be 'giving up' your copyright. Although my person interpretation of the above sentence would have been that it can't be subject to someone elses copyright unless I have their permission that is not what my made up example says...
The other thing with social media is the images and text content you are seeing may actually be supplied by the media. So the website may not have "copied it" they may just be distributing the social media's content. Not the case in the Gurnard's case but it can happen through web services and can still look very different. If that's permitted use by the Social Media site you can't argue.
So The Gurnard should check the T&Cs of the site he's uploaded them on to make sure the author doesn't have a case.