What is the total number of discernible still images that are possible to exist?

I would like to know how many uniquely discernible images are mathematically possible to the human eye.

We can take the amount of colors available on some HD screen to the power of its pixel count, but my question is more along the lines of what our eyes would recognize as different, without having to inspect and compare the images too closely or with tools.

I believe that certain colors are easier to distinguish than others, and perhaps different patterns as well, which leads me to wonder if an answer even exists at all. Maybe there has been work done on some estimates at least.