Why do we use the top left corner as the origin in image processing?
I propose that the raster scan is a Western invention, in analogy with the way we write: left to right, top to bottom. Remember that "rastrum" denotes a  rake. When you rake a surface, or paint a floor, one usually does this such that one does not step on the raked or painted surface. The "raker" or "painter" moves backwards. One wants to avoid the situation described in Painted Into A Corner:



This defines a sort of 2D causality, using data for above and if not, from the same level (row) but left.  Parsing the 2D space top to bottom then left to right could have been an option. However, we know that the human vision is more sensitive to vertical than to horizontal, as one can see from the JPEG quantization matrix. 

So perhaps, by going left to right first, the human mind could more easily spot a vertical change in a whole picture. *

However, the ancient slowness of a beam parsing a screen is not less important. For web browsing, it can be more important to first display a global low resolution patch and refine it iteratively, like in the progressive JPEG, JPEG 2000, or the clever Adam7 algorithm used in PNG.

See for instance http://www.libpng.org/pub/png/pngpics.html