Perhaps the rarity of this image could some how be represented to create a visual argument that aligns with the caption. This is an interesting problem because, with the caption, the image is metonymic of a lack of other images... but visual representations of lack are conceptually tricky. So I'm not exactly sure how to go about that. You combine the image with a pithy statistic, or perhaps include some of the metadata of the image that signifies its rarity? Or perhaps you could include a lot of blank space around the image to indicate the absense? I don't know if i like any of these suggestions in particular, but I hope there is potential for play in and around them.
I think some of the metaphor of having to really search the photo for the settlers actually is part of the ethnographic import of the image, but another tactic could be to actually blur the home, the picket fence, and everything in the photo except the settlers, actualling landing the focus on the people themselves. This might represent what you’re trying to do with the project as a whole. So it would be a sort of reversal of what is being portrayed in the photo now.