Whatever your imagination has created, the image has something different.
Images are not general. They represent individuals, that is, single things. Such things can be imaginary, like in this particular from Hyeronimus Bosch's "Haywain":

Yet, they have particulars, whereas symbols are general descriptions, which tell us how to build a mental image but allow a great deal of freedom in particulars. Middle age philosophers called such 'particulars' the 'contingent', that is, what can vary without changing the concept.
In fact, also the following image is "a beautiful black woman".

I think that the 'contingent' is the internal character of an image which makes us think that the image is true.  The 'contingent' is not predictable, because it is not implied in the concept, it cannot be easily imagined. It shows the presence of experience, the indexical character of the image: the world of individual and contingent facts, rich of infinite particulars, too many to be kept in mind.
The indexical image has some characters of experience. And experience is always richer that the concept, since concepts are instructions to build mind images with a lot of gaps that can be filled as we like.
Take the following photograph:

The load of metal is so excessive to be hardly believable if verbally described. It will be very difficult to imagine the particulars if you are not an expert in piercing.
Such a presence of contingent is an index of truth in images.
The contingent is even more when the photograph reports an ongoing event:

 

(the insertion of a "Prince Albert", a particular kind of male genital piercing)

A more sophisticated photograph. like this:

gives a lesser feeling of truth.

Thus, I think that the 'degree of contingent' is the most important internal character which lets us infer that an image is an index, i.e., that it is the effect of the real event which it represents.
Of course, the inference can be wrong, and the image can be completely artificial or synthetic. Or it can be retouched (roughly, indeed), by changing some pixels:

But cancelling  is easier than creating. The load of contingent does not change very much.
Reality, the experience, the contingent, are not surprising because we find in them macro-events that are difficult to invent or imagine (piercing is among them), but because it is filled with micro-events, a texture of small deformities, individualities, imprecisions, which cannot be produced by any synthetic imagination.

We understand, then, Japanes art, which does not aim to the concept, like classic western art, but is amazed by the contingent, and has a kind of painting whose main subject is just bamboo leaves. It is called sumie. In representing the leaves, the artist tries to imitate the absolutely casual way in which they are placed in nature. That is, contingency.

These reflections can help us in answering the question whether, in the time when any image can be synthesized, it still makes sense to talk of photography. The answer is 'yes', since what we find in photography is not 'inside' the text, but comes from our natural need to know reality.
 

Giampaolo Proni september 1997 - november 1998
 
 http://www.infotel.it/fabula/n-gpr.htm