I mean, if the point is something like this: there's no such thing as objective reality, so there's nothing for a photograph to index there are several problems with this kind of thing.
Immediately:
The first clause is open to discussion, but even if we stipulate that there's no such thing as an objective reality, it does not follow that there's nothing to index. Maybe photos index something else, after all.
At more length:
Even if we stipulate that there's no such thing as an objective reality, AND that there is therefore nothing for a photograph to index, NONETHELESS, the photo exists and is different from a painting. At this point we can start going on about levels of subjective reality, and point out that photos index or are otherwise attached to one level, and paintings another.
Because, ultimately, straight photos are not paintings. They are different and whatever the differences are, wherever they spring from, they are the root of what makes a photograph powerful as a photograph. It literally does not matter what model of reality, of thought, of whatever, you use, because photos are manifestly not paintings. Any model you use that mushes them together is therefore wrong.
Beating around the bush with philosophical koans doesn't actually change that, it just muddies up the water to no particular purpose.
ETA: The point I've been making all along is, in these terms, that people believe in an objective reality, and they believe that a photograph indexes a real slice of that objective reality. Even if we're all brains in vats, it is precisely these beliefs which give photography its distinctive power. If you like, you can delete all discussion of actual realities from what I have said, and replace it with equivalent statements about what people believe, and nothing changes.