Where is the implementation of the Cycles image-nodes sampling? 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

\scripts\addons\cycles\shader\


You want to look into:

node_texture_coordinate.oso
node_image_texture.oso


You will find there links to appropriate .osl or .h files.

In those you will find the code and implementation.