Where is(are) the function(s) that, in the image node, transforms a point in UV space (vector that goes into the node) to a color (color of the pixel of the texture)? I can't seem to find such. Thank you very much in advance.
What you are looking for is in
You want to look into:
You will find there links to appropriate .osl or .h files.
In those you will find the code and implementation.