The comma you're asking about appears between a subject (the course I did) and the verb that predicates on it (wasn't). Most of the time, you shouldn't put a comma in this position.
On page 1730 of The Cambridge Grammar of the English Language, Geoffrey Nunberg points out that we do find commas in this position once in a while, usually when it's necessary to prevent the sentence from being misread:
Most of those who can, work at home.
Without the comma, most people will read the sentence incorrectly. They'll get to the end of the sentence thinking who can work at home is a relative clause, and then they'll have to go back and re-read it once they figure out work at home is intended to predicate on most of those who can. Using a comma (quite exceptionally!) can help with this problem.
In your example, a misreading is impossible:
The course I did wasn’t even a pure photography one.
It's impossible for did and wasn't to combine this way, so we can tell that there's a boundary between them without a comma. The course I did must be a subject, while wasn't is the verb that begins the following predicate.
There is no justification for an exception here, so you should not add a comma in that position.
As an aside, you could optionally add a comma after the initial adjunct in fact:
In fact, the course I did wasn’t even a pure photography one; it was one area of optional specialism within an overall Graphic Design degree course.
But this sort of comma is purely a matter of style, and the author's choice to leave this comma out is perfectly fine.