Photographs seem to present a special problem in the signifier - signified - sign models.
For Barth ... I think he speaks to this in Image Music Text.
The problem might lay in Chandler's remark about lack of mediation: photographs by convention to not call attention to their mediation, and by convention the image is to lack mediation to be "authentic."
Peirce would place them as indexical and iconic (Chandler 38-43). But Chandler, via Grayson, remarks that iconic signs are read as feeling "closer" the truth (!) than indexical or symbolic. "Such signs do not draw our attention to their mediation, seeming to present reality more directly than symbolic signs." 41. Photos are indexical by virtue that they are an index of the light that fell on a scene. 42. Digital techniques are eroding this indexicality 42 - but indirectly, in my mind. That is, it's not digital techniques themselves so much as the opening up of digital resources as semiotic resources. Kress brings this up in Kress - Multimodality.
In typical viewing, photographs are not, in the end, seen as signs but as reproductions of realities. Perhaps more or less limited (in scope, in accuracy, in view), but those limits are not seen as semiotic affordances so much as technical limitations or choices made for aesthetic reasons. We can catch a trace of reading the photo as sign in seeing sepia photos as "olde tyme." But that sign is carried almost as a layering over of the image itself, which is still seen as a copy of reality that suggests little if any semiotic affordance. Consider Instagram filters, for instance. The filter is a layer over the image, but remove the filter, and the image is restored to non-mediated accuracy.
Which is to suggest that there are significant connections between Semiosis and Mediation. Perhaps, it can be phrased as: Semiosis begins when the interpreter understands the image as mediated and uses the semiotic resources of mediation to begin semiosis.
Photographs share some characteristics with paintings that are seen as realistic: portraits, still lifes, typically in oils or acrylics that present a surface w/o brush strokes.
chap 2: modality. Photographs and film have high cues: "'in film, the signifier and the signified are almost identical ...'" citing Monaco, 62. "In being less reliant than writing on symbolic signs, film ... suggest less of an obvious gap between the signifier and its signified" 62. - which also seems to short-curcuit semiosis by representing the signifier as signified. When this occurs, "The signifier is treated as if it were identical with a pre-existent signified and ... the reader's role is purely that of a consumer...." quoting Tagg, 64.
chap 4: the photograph might be seen as metonymic - the image as a standing in for what is represents. In this way, metonyms can place people, etc in relations that are not possible with the signifieds.