I have a brief break at work here...
I may be a little woo-woo (or a lot), but I actually have no personal problem with the idea that someone can go back in time and cause a physical effect. I don't consider time-traveling nonsense personally.
I've experienced time and reality distortions that would put most people in a straightjacket, conscious telepathy, physical PK, TP contact with previous-self, all kinds of crazy-sounding stuff. I don't make a big production of that because I can't prove it to others the way I can demonstrate RV, so why even start, why bother, this is something I occasionally blog and my friends and I talk about into the night, but not something I'd ever go making big claims about, simply because there's no way to support it to others.
But in terms of 'theory'--and what is possible--I like the ideas of both time-offsets and PK, in general, and I don't dismiss that someone might accomplish such a thing, by accident or design.
'willing' his initials or something into a photographic plate or whatever in the PRESENT before I'll buy any of this time travelling
This particular event bears the same burdens that all PK-based attempts/claims do, even in science, and it isn't really the fault of the experience, it's just the nature of the thing. That is, that you must be able to demonstrate that something has changed, and while this might sound straightforward, it isn't always so.
For example in the RNG research, they measure a period(s) of time and say, "Ok, this period was less random than others so the consciousness of people caused that." But the problem is that someone had to decide what period(s) of time to measure, and the innate psi of the researcher may cause them to make decisions that support that effect. There's a couple of white papers on this, called DAT or 'Decision Augmentation Theory'. Essentially in a nutshell it is about the concept that if psi is real, that the researchers running science are as likely involving it in their decisions, setup, subject selection and more, as psychic subjects are. There are protocol points/statistical ways to deal with this -- to predict when this is really the effect and to demonstrate when it likely isn't -- but nobody seems particularly interested in that alas... maybe someday.
But this issue is applicable even to remote viewing in some respects. For example, if you do a session precognitively, and then go randomly generate a target for it, is it entirely that you did a good or bad session, or that you psychically arranged the instant-of-request so that it influenced which became the target? These things are "mental models" that a viewer has to decide for themselves, based on what paradigm is most conducive to their skill and/or sense of comfort with existing belief systems. (Those are usually two opposite things.

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This dynamic is in bold-neon for any PK claim. In the present, if you follow a standard science protocol (which will have commonalities with a decent RV protocol), (1) something will be planned and documented ahead of time, and (2) done within a 'controlled' situation that precludes non-psi interference, assumption, etc., and (3) the effect, data, documentation, etc. secured, and then (4) later evaluated in the most objective way possible, and then (5) published with documentation and protocol inclusive, after sanity-check of course, for peer review, whether that is laymen on the internet or a peer-reviewed journal.
Unfortunately, as you note Marv, it is physically and logically impossible to prove that anything in the past has been changed. That doesn't mean it can't, it just means it can't be proved.
It is entirely possible that reality is subjective and we are all changing the past constantly, just (a) usually in smaller ways and (b) we may, on a psychic level, actually interact with others based mostly on 'the points where we agree' as this is less threatening to our sense of continuous (and contiguous!) reality.
It is also close to logically impossible to prove that anything in the present or future has changed unless it is done within a controlled protocol. This is important because this again impinges on an understanding of remote viewing, too.
This time-travel-PK-claim is in the same boat as "spontaneous psi". If someone tells you, "I totally had a psychic flash that was going to happen!" what can you say? It isn't remote viewing unless it's in protocol and that includes that it is planned in advance, done on purpose, documented before, during, and after, and that the results of the experiment must be able to be compared with the original and/or the feedback in order to validate that the effect/data is real/accurate. If any of those protocol elements are not in place -- or the situation (as in this case) makes it impossible -- then this becomes "a personal experience" but not something that science (or even other people) can touch or see reason to believe.
Personally, I would not have announced it in the way it was done. To me, it sounded like the locals were awestruck as a result of GW telling them about it personally; and this inspired one of them to go on about it on the internet because it was just so exciting -- where alas, people with somewhat less pre-existing rapport with GW had a different reaction to the claim.
I think in a field of social politics it sometimes helps to try and separate the personalities from the points. I find the retro-PK stuff very interesting on its own. This example is not well suited to that, any more than spontaneous psi is suited to remote viewing, but the idea in general is cool.
Edited to add this point: while equally unlikely, even if we assume that everything said about this is 100% true, there are still other explanations. For example, it is possible that the tasker's choice of target was subconsciously psychically based on a photo with an anomaly; and that the viewer's choice of lettering was subconsciously psychically based on an anomaly that was already there. Like I said, unlikely, but my point here is that unfortunately we just can't know either way.
The fact that one needs to alter the compression of the image in order to make out what must be barely discernible letters doesn't help their case either. Betcha anyone can find all sorts of words/letters etc in a myriad of online pictures given a decent zoom function and some recompression settings.
Hmmn. An interesting idea; reminds me of reverse-speech; might there be some visual analogy to that in digitized photographs?
QBL (Cabalah) would indirectly suggest that the universe is holographic and nearly everything carries patterns which potentially can be 'read' for information about something else. Probably many things we haven't even thought of yet.
Maybe the real lesson has to do with discretion and not making big announcements about stuff -- since in a way this is no different than the last dozen years of "psychic and remote-influence" claims made online which, I might add, even when made only as quiet discussion elsewhere, GW has been quite vocally against. This kind of puts them in the same boat. I see the psi as less a problem than the venue/approach.