Definitions

Technical code - What something in an image technically is, with no room for argument or discussion, for example, blue is a colour.
Symbolic code - a meaning or interpretation behind an image, for example blue represents sadness.
Signifier - The connotation refers to the literal meaning of something.
Signified - the denotation refers to the associations that are connected to the something or the emotional suggestions related to it.

For example, the picture above is of Batman and Robin. Technical code would mean that Batman is dressed in dark colours, and Robin is dressed in brighter, more vibrant colours.

An example of symbolic code would be Darth Vader. He is dressed in black with a red lightsaber, the black representing evil and the red representing danger.

A signifier is the literal meaning of something. As we can evidently see, without question, Donnie Darko is stabbing a barrier.

This is an example of the signified denotation - the audience will assume that Twist is sad due to his reserved body language and the fact that he is crying.