[kmmlng]

I would argue that in a signal (1D) you can expect some sort of relationship between consecutive elements. In an image (in essence a 2D signal), you can expect a relationship between consecutive elements not just on the horizontal, but also on the vertical axis.

If you arbitrarily represent a signal as a 2D matrix, then abrupt changes in the gradient on the vertical axis are meaningless. But the same is not true in an image, which is naturally represented as a 2D matrix. Here, a sudden change on the vertical axis usually corresponds to an edge in the image.

If you represent an image as a 1D array, you throw away spatial information. So I'm not sure about the 1D-ness just being in ones head.