There are, in very broad strokes, roughly two things that can happen in a group. One is Group Polarization, in which through a variety of interactions, the group's attitude taken en masse as well as individual by individual tends to move toward a single extreme position. The other thing that can happen is, well, the opposite of that.
I will quite likely botch some of the details in what follows. God knows there's a great deal of research into these things out there, so feel free to check me. Please do, at least before you quote me anywhere. As always, I'm trying for a quick sketch that captures the broad strokes.
There are three big motivators working here. There's the desire to go along with the group, there's the desire to express one's individuality, and there's a desire for increased social status. In a group with a more or less uniform set of attitudes, the easiest way to accomplish all three at once is to read the temperature of the group, and express that shared attitude in a slightly more extreme way. In the first place you're basically agreeing with everyone, in the second place by being more extreme you preserve your individuality, and in the third place with a little care you can turn this into a social status jump because you are clearly the Mostest Whateverest.
This leads to a death spiral, of course, if more than one person gets into the game. Then you all shoot one another, and Franco takes over despite the fact that you were winning just a moment ago. See also internet forums, comment threads, and political parties.
Group Polarization is easy to induce, and it's very convenient for anyone who wants to manage you.
Since people alter their attitudes in quite small increments, if you can keep your population polarized, then everyone is is neat little boxes where they can be managed. By polarizing them radically, you ensure that they stay in the box -- the edges of the box are too far away to reach in one or two small steps, and the ongoing polarization activities will chivvy them back into the radical center before they get near to switching parties. In the USA we have Fox News and NPR dutifully keeping their blocs neatly enbloc-d. Why the GOP wants to cut funding for public broadcasting is a mystery, it's easily the cheapest tool to keep the lefties pinned down ever.
Politicians are typically not very polarized, although they engage in polarizing activities more or less as a profession. They're pretty pragmatic. Ditto captains of industry.
Most political art, indeed most political action, is just polarization activity. Preaching to the choir is easy, it gets you accolades from your peer group. All you have to do is make some pictures and write a little text that boils down to "you are so pretty, the only error I find in your thinking is that you don't know just how pretty you are!!" which is a pretty easy sell.
Most political action is therefore utterly ineffective. It serves merely to keep lefties in the box marked Lefties and the righties in the box marked Righties where the state knows how to handle them. Your art, while helpful to the state, isn't necessary. The state has many many resources available to accomplish the ongoing polarization needs.
The only way to actually make a difference is to induce the other sort of group behavior, in a group and at a time when a useful change is a small step away.
You're not going to persuade a politician to make a radical jump. You're not going to persuade a bunch of racists to love their neighbors. You can with a bit of effort and a bit of luck persuade almost anyone to make a small jump. You can make the racists believe that a few of Those People are ok, kind of. You can make a politician whose vote on such and such was always in doubt anyways lean this way, or that.
Small changes, at the right moment, can shift a tide.
Gene Smith's Minamata essay appeared after Nixon created the EPA. Nick Ut's photograph was shot when the US troop deployment in Vietnam was dropping precipitously, and well below the peak half million or so. These pictures didn't create inflection points, but they did aid momentum at possibly critical moments. Politicians who were already more or less willing, but who might have dragged their feet, didn't. Or dragged their feet less.
Your photographs will not make White Europe embrace the refugees. It might make White Europe despise them less, and sympathize more. Your photographs might prepare White Europe for the work that next year makes things a little better still. Your photographs might persuade a politician already inclined to support pro-refugee policies that it would be safe to actually do so.
What about actual mechanics?
Points I've made time and again: Photographs derive their strength from depicting what is in a literal-minded sort of way "real." People looking at Art will try to fit what they see into a mental model, and in the case of the photograph they will try to fit it into a model that is "real" in some vague sense. Even if they're looking at clearly staged things, the viewer will try to imagine what the stage was, what the players were doing, and so on.
If the aim of your Political Art is to change attitudes, opinions, to alter the course of history by altering people, then it is this mental model fitting that you're going to need to use. I don't know if this is the only program that works, but here it is anyway.
You should have in mind, roughly, whose opinions, ideas, attitudes you want to change, and you should have roughly in mind what they think right now. So simply assuming that Trump voters are all three headed imbeciles isn't going to work very well, you'll need a realistic idea of your targets.
Photographs are going to work best on an emotional level, I think. Reasoning, making a logical argument, in pictures strikes me as difficult.
I draw from the theory of harmony in tonal music, for this next bit.
Show them photographs that they can believe are true. Do not over-challenge their world view, lest they dismiss your pictures as faked (in one sense or another: photoshopped, staged, cherry-picked, cropped to change meaning, and so on). The pictures must first build trust. Your target audience has to see things the "know" in an emotional way to be true. Then, ever so gently, show them similar photos that challenge that knowledge, or force the viewer to either reject the picture or expand their understanding of the world. If you're built enough trust, they might just expand rather than reject.
Perhaps you merely caption perfectly reasonable photos in a way that lightly challenges.
They know that war is hell, they've seen plenty of pictures of casualties, of explosions. Then you show them one more casualty, a little more dramatic, a little more sympathetic than before. It's a little girl, naked, running in fear. It shocks them, a little, but they've been prepared, they believe it. And then you caption it to reveal that it was us that did it, our allies did this. It was a mistake, but it was us.
Hit them emotionally, hit them in their identity, "we are or are not the kind of people who do this" is the message you want to implant.
All the rational arguments in world against the Vietnam War won't move the mountain. Neither will all the terrible pictures of war. Together, the rational mind is persuaded, the heart wants to follow.
Close with less challenging pictures, close with pictures they will find easy to believe. This is analogous to the way dissonance is handled in tonal music: bring your harmony along beautifully (i.e. without dissonance), landing on a chord which "prepares" for the dissonance, by being close to it in a harmonic sense, then "resolve" which a chord that includes the dissonant tone, and carry on the harmony beautifully (without dissonance) from there.
You slip the challenge in, wrapped in pictures that are obviously true. Perhaps accompanying text makes the rational case, but again in relatively friendly terms.
Don't insult, rather, find common ground.
If you wanna change the world, it's got to be one small step at a time.