Image

What is an image?

For me, it is first of all a language problem. If you look it up in a latin dictionary, imaginatio means 'a fancy, imagination'. That is not satisfying at all. If you look it up in a latin - german dictionary, you get the term 'Einbildung'. As Harun Faroki and Mary already mentioned, I would like to deepen that.
'Ein' means 'a' or 'one', but is also used with a inherent direction, like 'in' or 'into'. 'Bilden' has many meanings, like to form, compose, generate, develop, accumulate, frame, make, give rise to sth.
One could put it together as "something comes in our direction and we / our brain makes something out of it".

Roland Barthes wrote in camera lucida about the development of photography. He describes the process of light being reflected of an object, crossing a lens and manifesting that reflection on a film and then a piece of paper. When we look at that again, we still see that reflection. If you look at a picture of a friend, the light might have touched his skin, in a materialistic idea.
Coming to digital photography, we leave this component. Light loses its materialisic component and is reduced to the electronical character of a wave. This information is stored electronically again, and kept it the universe of data, to be shown on a screen or printed on paper. The reflection of light you see on that paper never touched the object you're looking at. It died. That's why digital images can't olden. They were never developed.