There are four main rhetorical principles that are the widely accepted modes of persuasion. I’m only going to give a nutshell view of these principles, they can be understood to be much broader than the simplistic way I’m going to talk about them, but this isn’t the place to delve too deeply into philosophic debates.
-ethos : The characteristic spirit or prevalent tone of a people or group.
-logos : This is basic logical reasoning: data, facts, words.
-pathos : This is commonly understood as a straight appeal to emotion.
-kairos : The application of kairos in photography can be best understood as finding the “decisive moment.”
What you are trying to do through these arguments is to allow your audience to relate to or identify with the subjects in the image. You want the audience to imagine themselves in these situations and even as the people depicted. It isn’t enough to merely show need, the audience should feel the need, understand the need and feel that they are not powerless in alleviating those needs.
Let’s look at an example of each.
ethos – Here we see a very typical African church worship service. This shows how much the people get into the worship how their churches are built. We learn about them through their environment and the way they choose to seek spirituality. We can imagine worshiping with them. We can feel their intensity as they sing.
logos – Often logos is easily done through detail shots to isolate the subject, to force attention on one piece of data. Here it is food, and the quality of it and the lack of it.
pathos – The straight appeal to emotion is easily the most overused principle of these. It is easy, it is direct , it is immediate and very effective. There are few things that can draw out an emotional response as children in less than ideal situations.
Because of the immediate direct impact, these type of images are so prevalent in the humanitarian aid and missions world. Because of this, they have become less powerful over the years as people have built up a mental wall to block their effects. The glut of these images has additionally had the effect of making people feel that the problems are so huge that they cannot do anything that would make any difference. Be careful when presenting these type of images, they can be both powerful and detrimental in motivating your audience.
kairos -No one has ever done this better than Henri Cartier-Bresson. Finding that exact moment when the action is peaked, or the moment of anticipation right before. This brings the viewer into the situation in an almost tangible way. It holds their attention and they are likely to spend more time examining the image.

A family plays in the snow in Pendleton, Kentucky. (Photo copyright mission photographer John Palmer Gregg)
These are by no means an exhaustive tool set to use in creating your argument, but they have been tried and tested for thousands of years, all the way back to Aristotle’s Topics . For more examples see the galleries ethos, logos, pathos and kairos.