when do you use the plural of a word? For example: Images through the internal auditory canals show / shows the 7th and 8th nerve complexes clearly.

The verb you want is "show"; i.e. the plural one.

This is a good question, because "Images through the internal auditory canals" could be considered a single concept. e.g.

Cows in a field is something you see often.

In this example the subject is "Cows in a field" and not "Cows". So, I think I see your point. I cannot find a rule to give you your answer, so I judge this to be a case of style preference.

If you used "shows" in your example, I am certain most proofreaders would change it to "show".